I haven't played D&D for a while though recently I have started playing it with my kids. The group is made out of five. So far they have acted with wisdom and they have been lucky which allowed them to slay the Green Dragon in Thundertree. Are there any rules for disassembling the dragon for the sake of doing things such as selling its meat and making armor from its scales?
[RPG] Are there rules for disassembling a slain creature, especially dragon
Yes, you are overlooking some major flaws
I'm only going to address the Major Flaws you're concerned with, since the rest is subjective to both your game play and how people play the class. This should help you get through at least a couple of misconceptions you have.
- Pact Weapon (PHB. pg 108) - You can only have one of these, and due to that, Thirsting Blade can only be used with your one pact weapon. If your DM does happen to allow you to take two pact weapons, that's cool, but is clearly a house rule. Make sure your DM knows about the wording of Thirsting Blade with respect to Pact Weapons.
- You have only one bonus action possible per turn (PHB. pg 189). If you have many abilities that use the bonus action (Cunning action, offhand attack, specific spells, etc) you have to choose which ONE to use.
- Survival wise - largely dependent on magical item availability, your feats, and potion use. If you take fighter at level 1, you'll have higher hit points to start, but your saves will leave you mentally vulnerable. That's why I recommend Archfey pact below, to remove that problem.
The following part is only my opinion on the build itself, and where I would make changes to better fit your stated style of play.
Personal build recommendations:
- Take 3 levels of fighter to become an Eldritch Knight so you can have weapon bond (two weapons bonded, functions very similar to Weapon Pact, allowing for 3 summonable weapons.)
- Instead of Fiend, use Archfey as patron because the up close abilities and immunity to charm are superior for an up close fighter. Especially since Misty Escape gives you a free exit in overwhelming situations.
- Feats - Heavy Armor Mastery, Tough.
There are no special rules for 3D distance
Emphasis on the special here. The default rules already assume a 3 dimensional world. Hence why areas of effects are described as 3D geometrical shapes such as spheres, cubes or cylinders rather than 2D shapes.
The DMG pg. 249-252 has some optional rules for estimating how many creatures are caught up in an area when using theatre of the mind or anything else than doesn't involve precisely figuring out where everything is in relation to everything else and some rules for translating ranges into square grids ("If an area of effect is circular and covers at least half a square, it affects the square").
In general however, the rules assume normal euclidean distance, so your guess of "about 35" is roughly right (36.something but it doesn't matter at this point). The DMG recommends using things like tape measures when estimating distances on a tactical map and otherwise just letting the DM make a judgement call.
It's perhaps worth noting at this point that things like grids and tactical maps are considered optional rules in 5e, while the PHB and DMG offer a little guidance of how to play using them it's generally expected that groups come up with their own ways of managing such minutiae.
There are no specific rules for disassembling a dragon or other creature to sell its meat or make armor from its scales.
Your mention of Thundertree leads me to guess that you're playing The Lost Mine of Phandelver, which implies a setting of The Forgotten Realms -- although not necessarily, you could forklift LMP into your own world and I am sure many do, and even if you want to remain as close to the default setting as possible, you end up having to make choices about how things work in your game.
I am not aware of any reference in 5E materials suggesting that the sale or consumption of dragon meat is a common, or even rare, thing.
There is official dragon scale mail armor, so it's reasonable such a thing could exist in your world. Making armor is a skilled activity, so maybe your characters have that skill, or maybe they don't. Maybe they need to find an armorer. Maybe any skilled armorer could make it, or maybe there's only one armorer in the entire world who knows how to make dragon scale armor. Xanathar's Guide to Everything (an official supplement) has optional rules that suggest crafting a very rare magic item would take 25 weeks and 20,000 gold, but it's up to you whether you consider that as reasonable in this case.
There are also numerous references to items made from dragon hide, dragon bone, or dragon scales which implies that there might be a good market for such items.
However, one does wonder how a trade in body parts of sentient creatures is perceived. Might a Dragonborn character take offense to a restaurant selling dragon steak? Might a dragon display the hide and bones of a human in its lair? "Yep, bagged this one back in '03, trying to sneak in to steal my treasure."
There are a lot of maybe's and might be's because this specific situation is not specifically accounted for in the rules. However, there is always the basic rule of D&D:
It's perfectly reasonable for the DM to say, "Okay, you've got a dead dragon", and for the players to then say, "Can we butcher it and sell it?"
Then, you as the DM need to decide ...
...what you want to do at that point. You can deal with it as simply as "Yes, you're able to sell some meat and scales for a 1000 gold", or "No one wants dragon meat, ewww, but the armorer in the nearby town will make you a nifty set of dragonscale armor if he can keep the rest of the scales." Or whatever you want.
You can make dining on dragon meat to be the height of haute cuisine, or taboo, or anything in between. You can spend a great deal of time focusing on what to do with the dragon carcass, or you can dispense with it quickly and move on to something else.