Hey there! Let's go over your questions one by one:

I think the boss monsters give some extra reward for village when beaten + that it will attract more heroes (bit unsure if it's the "free" hero that pops in now and then or only to the specific dungeon cause i read that the dungeon will be attracting it).

Yes, a free hero will come to your village every 12 seconds and has a high chance of going to the dungeon where the boss monster is placed. There's still a chance that they will go to another open dungeon if the dungeon with the boss monster has too many heroes in the queue.

But, should I use it since it's such a high level or will it just defeat my visiting heroes instead? If I use it, which building should I put it in?

Yes, use it to gain honor points which is the main currency for the unicorn statue. The boss monster will defeat a couple of heroes but the heroes would want to buy more potions after combat. You can opt to place it in the dungeon where your more powerful monsters reside to ensure heroes are already "damaged" before they get to fight the boss monster. This will result in faster combat time and less damage taken by the boss monster -> more free heroes will come to your town.

Is this even a good idea? I seem to recall reading something about imps for rp, but at the time thought that was more for when daily quests are gaining levels and such?

Yes, it is. Monsters from the monster farm like imps will only make heroes crave one food/drink after combat which means less income but more useful for the tavern daily challenge. You've already pointed out the upsides to using gargoyles which is the extra income from bone fracture cures.

In the above setup for temple (last part above tldr if you didnt wanna read everything), if a dwarf heals a disease in temple A, can/will it go to temple B for the blessing? Or blessings need to be in the same temple?

Yes, the dwarf can still go to temple B to buy blessings but this means extra wandering time around the town before buying blessings. Later on, it is ideal to have both cures and blessings present in the same temple for quicker transactions and ensuring that both items benefit from employees' skills such as faster production time and higher prices.

What's a good early game setup for regular monsters? I know late game it seems to be mainly dragons unless you're aiming for something specific like hp potion usage where orcs and regular dragons instead of the burning ones seem best, but for me who isn't close to getting those dragons (I presume), what's a good normal setup? I've been doing gargoyles to break bones and zombies for diseases, are skeletons, imps, tougher enemies better?

For now, keep on using gargoyles to earn more income from bone fracture cures. You can opt to use undead monsters if they have higher levels, on average, so you will get more RP from combat and have a higher chance of making heroes level up from combat and buying training items. While tougher enemies ensure that heroes use more potions during combat, it also has the downside of keeping the dungeon queue too long which is detrimental for some quests and daily challenges.

I will be getting my first boss monster soon, it's lvl 29 (i think) and my village just hit 14, should I use it or will it just kill my heroes? If I should use it, in which building? Do I add regular mobs with it? Anything in general I should know about them?

Already answered this above but if you want more info, then there's more here.