Bach Party Advice

Maybe talk to each girl privately, address the concern you've seen over group chat and kind ask them to hold off on any drama until after the Bach party. Im sorry you're going through this, it can be so awkward!
Also just wanted to comment that I love mackinac island! FH proposed at Arch Rock last summer so that place is even more special now. As far as cheap entertainment there, definitely go fudge hopping and take advantage of all the pretty walking trails rich with history.

I hope that they know for your day they'd keep it civil for your sake and enjoyment.

If you want to avoid most drama you are going to need to pull each girl aside privately and tell them the concerns you have and that these actions will not be acceptable during your party nor during your wedding at any point. They are there for you and need to put their differences aside.
Secondly, for your friend who is on a budget I see two options. 1) you change your idea to match her ability to pay and maybe find an equally fun, but less expensive place to stay to be able to include her or 2) you accept the fact that she is going to have to turn down the trip not because she doesn’t want to support you, but unfortunately she is not in a good financial situation to be going on expensive trips.
It’s really up to you at the end of the day. What do you want your party to look like? Either way you are going to have to be confrontational with the two girls or your not going to get what you want no matter what. It’s unfortunate that they can’t just deal with each other in honor of your special day, but some people let things get the better of them. I hope everything works out for you.

Your friend that can't afford to spend money probably shouldn't go on the bach party, or you should pick a cheaper location. The main person planning really should have checked everyone's budgets prior to planning.

I think I should clarify a couple things,
1: I paid for the hotel, me and my FH spent over 500$ on the hotel room. We are not asking to be reimbursed.
2: transportation to and from the island has also been taken care of.
3: I paid for her bridesmaid dress
4: food and drinks are the only responsibility.
I planned my own bach party because it’s a popular time of year so it was booked last summer.