A Couple of Questions Regarding HG/SS

Even though I'm familiar with ROM hacking up to Gen 3, I recently took some interest in HG/SS. However, I would like to know if it's possible (and how easy or hard it is) to do some changes that I want for my hack:

- remove the following/walking Pokémon feature
- edit trainers and wild Pokémon data
- edit evolution trees/requirements

Thanks in advance.

As far I'm aware it's possible to do only the last 2 points, which can be done quite easily with either PPRE or Spiky Pichu's Editor. For trainer related shenanigans I suggest you to use the 2nd tool since it allows you to do more stuff like giving their pokemon seal ball animations and even change their abilities for the HGSS games.

The only changes I would do on trainers would be to change their Pokémon and respective moveset.

So, it's not possible to disable walking Pokémon?

Yeah, in that case any of the 2 tools should do the trick.

About the second question, so far no one has ever done but it should technically be possible since there are specific areas in the original HGSS that disable the following pokemon feature (like Clair's gym iirc).