This is stretching on for longer then it has to, but I still have a few more things.
Can I use a .cro that I made for Y in a X patch?
I don't think so, just did a quick comparison with the dllfield.cro files and they seem to be different. Probably best to have separate .cro files for each version.
How can I edit the title screen (PK3ds title screen editor doesn't seem to work).
I don't know this one sorry, someone else made the title screens for EXWY.
Can I force the player's gender?
This would be an ASM hack (or maybe a script edit, not sure), so that depends on you. I definitely couldn't do it!
How can I write/edit scripts?
There's a script/map editor for ORAS (look up CTR Map), but it's not the most beginner friendly program and it wasn't made with XY in mind. You can give it a try but I can't help beyond that.
Some garcs (like pikachu's texture in a/0/0/7) can't seem to be extracted, is there a way to fix this?
You could try other tools maybe, but other than that I have no idea.
And finally, do you know how I could put pokemon from Sun/Moon into X/Y? I would like to put the Gen 7 starters in my version.
I don't think this has been done yet; it's not a trivial process (can't just copy/paste all the relevant garc entries from SM sadly) so I can't help with this one either.
Again, sorry for all the trouble, but I just can't find any of this information anywhere else.