The DM decides what campaign he runs
While it's true that roleplaying is a cooperative effort, and it's better to find a consensus... Your campaign is yours. It's your world. You decide how it works. So if every ork is evil in your campaign, then it's so. Your players may not like it, but they're not the GM, so there's that.
That being said, while your are certainly free to decide what your campaign is, if none of your players buy-in then you'll be left without players. Try to find out why they want to play a good tiefling or whatever.
Is it mechanical benefit? Take a similar good race and be done with it. If tiefling aasimar is a great option.
Is it love to that specific race? Maybe you can make them half-blood, maybe a spell changed their alignment for good, maybe the stars aligned on their day of birth. They are the solo good aligned tiefling in the world. Every other tiefling should die, and they know it. Resume your goblin-slaying antics.
Is it because they don't want to try something new, or don't think playing the campaign you envisioned will be fun for them? If that's the case, try to focus on telling them why you want to play your campaign and why letting them playing "evil" characters would hinder the experience. If they're in, go kill kobolds in the name of justice. If they're not, you either make a different campaign or find other players. Maybe convincing the undecided to try a first session and making it awesome can help seduce them into your campaign.
Is it because they want to play out the struggle of having an evil nature and a good heart? Well, that's clearly not what you want. Tell them that this will be dungeon-delving monster-slaying and their sentimentalism has no place in your world.
On a final note, in D&D5e alignments are there mostly for historical reasons, it seems. There are fewer mechanical effects than on other editions, no rules to govern when someone changes alignment and a general "flexible alignment policy" in place. They are explicitly told in the player handbook that they can freely choose their alignment. So if in your campaign alignment is rigid and important, say that clearly, because it's not the default setting.
Your players are invested in a different game.
The first group that I played with played the way you describe: mucking around town, not caring much about objectives, messing with every NPC they meet. It took us entire sessions to get through even the most basic of quest-giver scenes, and we delighted in spiking plot hooks and keeping the DM on his toes. I still play with that group, and it's still a lot of fun!
It is likely that your players have a very different view of the gaming experience--maybe socializing or being "off-task" is just a form of relaxation. While you may want to push through the prewritten campaign, maybe they want to just blow off steam and mess around in a fantasy world, doing things they'd never do in real life. Both viewpoints are perfectly valid ways of approaching a TTRPG.
On the other hand, the group I DM for is very plot-oriented---they will eat up quest hooks, and they will efficiently pursue their goals. When I first DMed for them, I was surprised that they completed a quest in the time it took my other group to stroll through town.
Adapt or pass the baton
Trying to force those players into "focusing" on the game will likely just swap your roles: you might be satisfied, but your players might chafe against your "railroading". The first DM of my first group tried to do this, and it caused a lot of friction in the group. Eventually, that DM became a player, and we had a different DM. This DM, instead of trying to run a coherent plot, simply had an omnipotent, insane NPC wizard teleport us around and told us to cause chaos. As you might imagine, this went over a lot better.
Additionally, instead of making deep dungeons or storylines, he focused on making more interesting NPCs and environments instead. This NPC focus meant that he was prepared to create interesting scenarios when we were messing around in town, and he didn't waste time on dungeons we would never enter. For example, in your tavern, you can have a champion or something challenge the PCs to a 1:1 fight, or give the shopkeep a interesting prank magic item (horn of baubles, maybe?). If your players like wandering off or doing individual things, plan more individual encounters or ones that don't require the whole party to be present. I realize you're trying to run a published adventure, but maybe that's not suitable for this group. You will probably get more mileage out of the adventure by using its encounters and NPCs as inspirations for your own modified campaign, rather than running it straight.
Basically, the three questions you ask at the end are basically, "how can I force my players to play how I want?" Instead, you should be asking, "how can I adapt my campaign for these players?" If the answer to that latter question makes you not want to be a DM anymore, then you should pass DMing responsibilities to someone else.
Best Answer
Short answer: stop playing with people who abuse you
You are being picked on. Sometimes, when something bad happens to us, we go into denial. We don't want to believe that bad things are happening to us. We don't want to believe that we are the butt of the joke.
RPGs can bring out the best in people, and, sadly, can also bring out the worst (been there, done that, etc). Since RPGs are a form of escapism, built into some of them are strands of a plausible deniability premise that goes like this:
"It's just a game, don't take it so seriously."
"My character would do that, what's the problem?"
But here's the reality of it: no character can perform an action unless the Player (the person) attempts it or tells the GM that this is what they are doing. Sometimes, the above quoted defenses are used by bullies to avoid culpability for being a jerk.
Your "friends" are bullying you.
You can either keep hanging around with them, and keep getting bullied, or you can find different and better people to hang out with.
Or ... the answer with a bit more risk ...
... before the next play session, you can call them all out on it before play starts. Begin the conversation, in your own words, with something like this: Why are you all doing this rude crap to me?
Some of them may not realize that they are being jerks, and some may be doing it because they are jerks. You won't find out the truth by asking on an internet site. You'll only find out when you ask the people with whom you are playing why they are treating you this way. Once you ask, the answers may surprise you. Sometimes, clearing the air improves the breathing. Sometimes, it reminds you of why we leave a room when someone farted.
To quote Cherlindria as she speaks to Willow Ufgood:
The choice is yours.