Well you have a lot of differing advice, but here's mine too I guess! I think that at 2 years old, the party is mostly for the parents and there is nothing wrong with that. Not that your son won't love all the attention, toys and cake, of course he will, but if you want to do something then do it. If you don't then don't. I personally love planning my kids' parties and do themes and all the good stuff.
Here is my advice. If you don't want to spend the money on a party room then just invite all the friends and their kids to the museum and pay everyone's entrance fee. Then find an ice cream place near by. Tell the friends that you are going to ice cream place for dessert at a specific time. At 2, all the parents are going to stay anyway, so it's not that big of a deal to leave there and go for ice cream. Tell your family that is the plan and please to meet you at the ice cream place. Then say that if anyone would really like to come to the museum then to let you know. I'm willing to bet that the other aunts and uncles won't come to the museum. Grandparents might and that's fine.
I would give yourselves at least 1.5-2 hours at the museum. I do think you need to be prepared to pay for everyone's ice cream who shows up though and honestly, it might be cheaper and easier to just pay for the party room. And that's my 2 cents!
Have fun and do what makes you happy!