ok. so, you want the money/need the money but you don't want her to take over, right?
Soooooo why not tell her the arrangements NOW, before money changes hands. Make it clear that these are the plans.
Suggestion - guest list of 100, take 50 each.
You decide 50, then your dh gets the other 50, and if he wants to let his mum invite her mates, it's sorted.
And the other thing to remember. It's just one day. That's all. What is really happening is that you are getting married. Have a marriage, not a wedding and these things will seem less The Be All And End All, iyswim. His mum wants 10 guests, it's no big deal. Take the control by saying we have arranged to let you invite 10 of your friends. You are letting her, you are in control, she gets her mates.
And I'm waffling....
Good luck, congrats and try to not get so wound up about it all.