As for not receiving any information about the final plans, I would think that since the wedding is still 3 months or so off that those plans might not be finalized yet. I'd ask again in a few weeks and then if you still don't get the information tell her you are going to make alternative reservations on XXX date.
Good grief, about everything else...3 months is plenty of time for people to ask for a couple of personal days off. Hubby included.
I think leaving the baby at the hotel with the grandmother would be ideal. Take her with you on the trip, get her her own room so you and hubby can have plenty of room for all those extra's that will be there too (wedding apparrel, baby stuff, flowers). Surely she can come along and keep the baby there during the ceremony. If you need to nurse during the reception you can make a quick trip to the room and take care of it. Find out where the wedding is being held and reserve your own rooms very close to where the reception is being held. That way if your son gets cranky or tired he can go to grandma too.
If grandma is truly too valuable to her company to take a day then by all means take a friend or hire a babysitter to go with you. Maybe a young woman from your neighborhood that is trustworthy. A teenager might be able to handle a baby and your son if needed too but I'd still take a friend of my own or an older adult who can handle any emergency. You and your friend could go have a fun lunch and maybe some do some shopping. I wouldn't think a friend would require payment, just time together and having some fun.
My BFF and I do a couple of roads trips together each year. She will often stay in the hotel room and nap while I take the kids to the pool or shopping, even to visit my family. She will take the kids to the pool or out to eat at McDonalds while my hubby and I have a quiet dinner at a nicer place. There are 7 of us, 2 of mine, 2 of hers, her, and my hubby and me.
It works due to us having understanding of what is expected.
I would approach who ever you ask to go with you like this:
We will leave here around Noon and arrive there about 2pm Thursday. The rehearsal dinner is 20 minutes away, and at 6:30pm. We need to start getting ready by 5 so we can shower and dress. You can do what ever you want until 4:45. Please be at our room by then so you can take the kids by 5. We plan on being back by 10 pm and after that we won't need you to babysit until 3pm on Friday. That evening is going to be long and we can't say for sure when we'll be back. Please call us if you need anything.
Just an example of what it might be like. The guest you take could also be someone you trust to drive your vehicle so if they wanted to go somewhere they could while they have free time on Friday. Of course my friend and I do stuff like the splash parks, skateboard parks, BMX races, water parks, all kinds of activities together when we travel but it's not for something like this.
Still planning something fun for the whole group could be fun too. Since the wedding is so late you could all spend Friday night too and then have some big activity on Saturday then a leisurely drive home by dark.