have there been any positive changes in this situation thus far?
i think it's wonderful that you're doing so much to support your daughter's wedding. no doubt you love her very much.
i just came across this thread for the first time, but i am wondering if your very involved role in this could come to bite you later on. you definitely don't want to get between a family and his son, even if it's only in their eyes. i think it's fair to have a budget. few people don't need one. you should be explaining your financial contribution to your daughter. it should be her responsibility to communicate these details to her fiancee. if you and her fiancee are very close, you can have this conversation together.
as i see it, the next step shouldn't be for you or your daughter to take. i think that her fiancee should be the one speaking to his parents. if there are issues, i'll bet there will be much less likelihood for the parents to hold a grudge with him than there would be if you or your daughter said something that made them feel like they were not being considered equally. sometimes it has more to do with perception than anything else. do not make them resent you! you should be bonding, not fighting over the details of a party.
something else comes to mind. i realize that you're paying for the Philadelphia wedding, but i was pretty surprised to read that you and your husband have decided on the first 80-90 guests, leaving what's left for the bride and groom. 80-90 people is like a wedding unto itself. that's a whole lot of people for them not to have chosen independently. i understand the family side, but are the friends your friends or their friends? unfortunately, few weddings can include everybody that's significant in some way or another. as this is really about the bride and groom, i think that they should be the only ones to decide who should be invited. you can sit down with them to explain why you think so-and-so should be invited as well, but don't "make" them respect your preferences, use intimidation, put them on a guilt trip, or threaten them on the money side because you're the ones ultimately paying for the reception. what you're offering is wonderful, but (at least in my opinion) it should be given as a gift to them, not riddled with red tape or booby traps. weddings can get that way. this should only be a happy occasion.
lastly, if the other parents want to experience downtown Philadelphia, perhaps it would have been a better idea to plan a time just for you and them to see the sights and so on. that way you wouldn't have to spend significantly more to take all the guests into the Philly experience.