Can I make the code for the minecraft classes clearer to me by adding comments, renaming parameters and local variables (but never renaming types, fields or methods)? Will that break something? like recompilation e reobfuscation or something else?

it won't break anything, but its just that whenever you reobfuscate that files will be in the reobf folder. you can just remove them again ofcourse so that the user just uses the normal once.

Oh... And is that avoidable? I noticed some bat files like updateids, updatenames, updatemd5. Would any of those fix that thing you said?

comments are 100% safe as the compiler strips them when it compiles, as for variable name, youd have to check thigns about the srg file to add naming to it... i dont know much about it, so thats pretty much the only hint i can give you :\

I'm not 100% sure of this, but I think those names on that srg file are for types, fields and methods only. I don't think there are parameters or local variables in there. I'm just guessing (it would be extremely painful to actually search for any local identifier in that huge file) but I think it makes sense. Anyway, unless theres a way of avoiding what OwnAgePau said, I think I'll leave local identifiers as they are. At least now I know comments are safe.

you can always change local variable inside the method you redefine, just not those in vanilla class