Re: What should I do???
I can see your dilemma-it's never easy keeping both parties happy.
Like others said a female only do should solve the mingling issue.
As for the rituals that are purely based on other religions have a word with your family and suggestion all the girls apply a bit of mendhi on you-that way everyone is included but it's not the 7 happily married women ritual (does that make sense).
I'm assuming your future husband will have an issue with music/dance. Dance wise if you do keep that element in your mendhi get people to do a loodhi (the group dance with sticks). It'll be fun plus it can be preformed to no music too.
If women want to sit around and sing they sound always use a daff (is that how you spell it?!!!) the drum with no tinkle that is allowed in Islam.
Hows about adding other things to the mendhi that will keep people occupied and keep your other half's family happy (if you invite them)? How about games that can be jazzed up-pin the tail on the donkey can become pin the plait on the princess, pass the parcel can be done to some sisters who may wish to sing without muslim and so on...hope that makes sense.
Good luck and have fun.x