CrystalConix

I'm not sure if download links would still be up but if you check the skeetendo forums you might be able to make an oldschool gold hack with tools, it's definitely not nearly as easy to use as pokecrystal but might be your only options. Otherwise Essentials is an option but be prepared to put a lot of time into the engine just to make it look like gen 2

The instructions on Github are fairly clear, just follow them to a tee. I dont know what else to suggest to you.

You dont need tools since its source code, but Polished Map makes mapping much easier. Just need windows paint, notepad and follow the instruction on github

I would say pokegold/pokecrystal is your best, easiest method.

Not really

If its small with some revisiting of earlier places you might just need to take some time to learn how flags and events work but sounds quite doable, especially if you dont have to worry about TMs. If it's small though you could probably settle for a smaller amount of Pokemon, it will make your life easier for movesets and such now that you dont have to worry about TMs

Depends on motivation on it, what kind of features you want and how big the scale of the game would be. Orange has been in dev for almost 5 years now.