You're not having fun.
Since the entire point of role-playing is to have fun, you need to do something about it. In particular, it sounds like the character he's playing (called a GMPC) is seriously distracting him from his GM task of running a fun story for the other players at the table.
You have several options, depending on whether you're willing to accept him as a GM as long as he changes, or if you can accept him as a part of the group as long as he isn't GMing:
- Talk to the GM about his behavior. Try to be non-confrontational, and don't have this conversation in front of the group; that will just make him defensive and won't accomplish anything. Try persuading him to not bring a character along with the group; he's GMing, not playing.
- Sit out the current game, until the GM is done. There's no need for you to continue playing a game you dislike, so take a break until he's done running the game. Then join the next game, that someone else in the group will run.
- Take over GMing duties, with the support of the rest of the group. Since they already asked you to be the GM, they'll almost certainly be willing to let you run the game. Talk to the other players about this individually before you try this; if you have enough support, you'll be able to usurp his position as GM, and start an entirely different game. Perhaps the current GM will be a better player than they are a GM. And look on the bright side: you now have some fantastic examples of how not to GM, which will make you much better at this new role! You can also combine this with #2 above: sit out the game until it's over, then be the next GM. (Warning: given what you've described of his personality, the current GM is unlikely to take this well.)
- Start a separate gaming group, inviting whoever is willing to play without the problem GM. If some people are unwilling to play with the new group because they don't want to exclude the current GM, then you'll need to exclude them as well. Recruit a few other players from elsewhere if you need to.
- Find a new gaming group. If all else fails, just don't play with this group at all. If they're willing to play with someone they dislike, and you can't persuade them to kick out a problem GM, that's their problem. It doesn't have to be your problem.
Note that none of these options include playing a game you dislike with a GM who you hate. He isn't going to improve on his own, unless you talk him into changing or encourage the group to leave him behind.
On a slight tangent, see the Geek Social Fallacies. It sounds like some of the members of the gaming group are suffering from GSF #1: "Don't exclude anyone" and GSF #5: "Failing to invite someone is a deliberate snub."
I know you're not a native English speaker, but in many circles of general nerd culture in America and on the Internet, there is a phrase for what you're doing right now: your spaghetti is falling out of your pockets.
In all seriousness, though, you seem very anxious about your situation and you should take a step back and relax. You say you're good at improv, but it looks like you're already sweating bullets because your party got sidetracked instead of getting to the quest you wanted to take them on.
I'm going to put my direct answer to your question first and some general advice afterwards.
Your players probably won't miss their characters. Why? Well, you generated them, and even wrote up their backstories. These characters were never made by the players playing them, and while they might have grown attached over a few moments, I find it extremely unlikely that they'll shed tears over a party that lasted for four sessions... that they didn't even make themselves. Even if the characters are extremely cool and interesting, that element of personal attachment just isn't there, which allows the players to truly say that they helped create an exciting story with interesting characters. This obviously varies from person to person, but again, I find it very unlikely that your players would have grown very attached in this case.
Go out with a bang. You know that amazing epic encounter you were saving for the climax of the plot arc? Yeah, run it now. You'll have to make some tweaks because you obviously aren't quite there yet, but get them there as fast as possible. Feel free to kill off PCs or even have a TPK at this point; memorable deaths are often much better than "and then they lived happily ever after."
Here's a few things to keep in mind when starting your next campaign:
Your content will come to light eventually, and it will be good. The quests you've designed will always find a way to come forward. Even if your plot arc is entirely ruined by something the players did, you will be able to recycle the content you made but never ended up playing, and I encourage you to do so for your new campaign. The only things that are truly lost are "hard" materials, like NPC stat sheets, etc.
You're in control, and therefore, you set the tone. Sometimes, it is best to take a page out of Gygax's book; after all, this is your campaign, and you put a lot of effort into it. Obviously you shouldn't take the entire preface from the AD&D DM's Guide to heart, but there is a point where a DM should draw a line in order for there to be some kind of structure, assuming you want your campaign to go anywhere. If your players are goofing around and killing NPCs for no reason, or making light of important people in-character, then they should be ICly punished for it; reprimanded for insulting a nobleman, pursued for attacking innocents, etc. It is also very possible to play a serious game in character and laugh until you're blue in the face out of character. This frequently occurs in the Dark Heresy games that I've played and ran.
You had better get used to murdering your darlings. This is a phrase commonly used amongst writers and creative designers everywhere in the U.S. The phrase means that you'll have to scrap ideas frequently, including ones that you really, really liked, so you had better get used to it. The saying is intended for use in the writing, film, video game, and other industries where a publisher or producer oversees your work, constantly telling you what can and can't make it to the final product based on time and expenses. However, it works just as well for when your ideas can't make it to the game because your players did something insane. And, on a related note...
Plan less. I don't know how much effort you're putting into writing everything ahead of time now, but you might want to ease up on that. From what you're telling me about your role-playing experience, it seems like you've been playing in a "safe" and slow environment where you rarely, if ever, have to scrap or re-do material. This happens literally all the time in regular tabletop RPGs, thanks to the insanely unpredictable nature of 4-5 different people working together. It will save you a lot of anguish if you lay out a basic outline of what's going to happen and then add the details once you're sure the players will be arriving there next session, or maybe two sessions later.
It seems like you've learned a lot already OP, which is great, but scrapping a campaign after 4 sessions (and while your players are all enjoying it) is something you should really avoid. If everyone else is having fun, consider either shaking things up a bit and changing your own notes, or coming up with a way to set them back on track, which doesn't always need to feel contrived or railroad-y.
EDIT: Well, now that SevenSidedDie has made that edit to your post, there are a couple of details that I didn't quite catch before, no offense. Since you said you have a month between each session, it seems like you might be over-planning because you have a lot of time between sessions. Heck, you might even consider having more frequent sessions, if you can't stop yourself from overthinking it in the intervening months. If in-person is not an option, use Skype and/or Roll20.
Best Answer
If you aren't an experienced DM, don't try to be Matt Mercer
Grow into your boots as a DM.
We have some other answers about this, but the advice remains the same. Since you are a new DM, play it straight for a few sessions until more of the people at your table have a good feel for the game. Then, with that foundation built, go wild and try Matt Mercer's house rules since now you know what they are a variation on.
Also note that Critical Role is a show made for a particular reason, and is peopled by professional actors. His house rules come from his experience of the RPG game form.
As to "am I acting reasonably" ...
You are going through a normal small group dynamic
You are in the forming and storming stage of the four step "Forming, Storming, Norming, and Performing" process that is common to small group activity(RPGs being such an activity).
Don't give up, work with the other players to come up with the set up that gives you all the most fun. It doesn't matter if you are the DM, or if you rotate as DM.
After each session, talk to each other.
It is healthy at a table when the DM listens to the feedback of the players, and when the players give constructive feedback to the DM. (Don't forget this when you are a player). In general this is covered by addressing:
What went well?
What didn't go well?
What did I like?
What didn't I like?
With that in mind, discuss all of that and then agree on what will we do differently in the next session? (If anything).
Play. Have fun. (It's why we play).
About that "magical sniper's eye" the player wants
A general theme in D&D 5e is that power upgrades can come through items and new abilities, and that they have to earn upgrades either by level advancement or by finding treasure during their adventures. Your group started at level 1. The game does not start with people owning magical items. This published uncommon magical item fits that player's desire:
The player needs to, in character, find or trade for this item. Or seek one out. Ask the player how they intend to find one.
As the DM, you can use this item as a plot hook for an adventure that requires the party to overcome a monster, or do a wizard a favor, or overcome some other challenge with this item as the pay off.
What he wants to do with it can be addressed with a feat. (Feats usually cost a choice at level up (foregoing an ASI) but there are some tables where the DM starts everyone with one feat each. Your call as a DM).
For longer range shots than are in the standard weapon's range, a player can choose the Sharpshooter feat. That costs the player a choice, but if chosen, the player has a 600' range with the longbow rather than 150' range (without incurring disadvantage); partial and 3/4 cover are negated, and in some cases a bonus damage feature can be chosen. That's a sniper, D&D 5e style.
The Sharpshooter feat (PHB, p. 170) says:
In D&D, in this edition, magic items and abilities like what he wants are usually earned, or are paid for with a choice or an opportunity cost.
Explain that to the player, if need be.
Uncommon magical items are typically found during Tier 1 adventuring, which is between levels 1 and 4. If he wants that long range magic eye thing, let him earn it through adventuring. That's part of the fun of this kind of adventure RPG: increases in capability through playing using the core mechanics of level ups, class features, feats, and DMG controlled magical items.