First of all, thanks a lot for playing and glad to hear you enjoying your time with it^^
A lot of things are still missing tutrorials currently unfortnutely. So to answer your questions:
1. It does a few things that arent explained in game.
-It cause you to gain more corruption points when getting a tf part belonging to that form
-Mind corruption also counts as bonus progress towards the tf form, making it easier to gain the tf's passive traits and also influences the bad ending when loosing to corruption.
2. After playing the intro or interacting with a save flame you should get a status effect book that you can use under key items. Most status effects are listed there with pictures and explain their effect. Currently some are still missing but most are expkained in there.
Hope this clears some things up^^ If you should have any other questions feel free to ask.
And happy holidays!