OK, you need to pick your battles here. I agree it SUCKS, but you need to keep your cool if you want to keep this experience enjoyable. And fiancee had better be on board. Maybe if you show him you are willing to bend a little, he'll take a stand on the more important things. that is marriage so he/you had better be ready!
1. keep your welcome dinner
2. you do NOT have to pay the site fee for off site guests, no way. I would wait it out and see how many of these "friends" rsvp before you approach how to fit them in your budget. They may not show, and if they do it will be time to talk to MIL about footing the bill. But give it some time so you know what you are dealing with.
3. Get your own cake too. If she wants to bring a cake, so be it. But have your own cake the way you want it for your pics, etc. Your guests will only benefit from 2 cakes! Don't even tell her that.
4. Let her provide alcohol for the OOT bags if she wants. Good luck transporting it! The rest of the bag, do what you want. No biggie.
I hope this helps. But by playing it cool you will avoid some unneeded drama. By giving in to some of these smaller things that can be worked around and including her in some decisions, hopefully it will detour her from some wild ass idea, KWIM? Play the game sweetie. But, your man better learn what side to be on in the end:)