I agree that this is your day and you should do it however you want it. I am sorry that your FH's family is stressing you out over this.
However, I attended a wedding with a similar scenario. There was a 5 1/2 hour difference from the ceremony and cocktail hour. I felt insulted by this because even though we were in a large city where we could sightsee, we felt like we (the guests) were all an afterthought for their wedding; our time or what we did with it did not matter. I believe that to a point this is a bride and groom's day, but I see it as this is the first major party/event you throw as husband and wife, and that your hostessing skills should come into play here.
As for the wedding we attended, we were already in our nice outfits, so we did not want to go back to the hotel 1/2 hour away from the church, change, sightsee, come back, shower, change again, and take public transport for 45 mins to get to the cocktail hour. So along with some other guests, we went a bar near the church and hung out there until it was time to leave. As a guest, I felt incredibly inconvienenced. I understand now why my FILs say that we have to keep our guests in mind for our wedding day. I know some people who took money out of the envelope because they were so displeased with the day's timeline. The B&G never even got to half their tables to say hello, but that is another thread
That being said, I think I agree with your FMIL to push the cocktail hour to 5 instead of 6. Your guests might appreciate that. Will it take you all 3-4 hours for pictures? If so, is there a way to include your guests on a tour of the city, maybe? Invite them all to tag along somehow, and have them take pictures of you all or the cityscapes? Or at least give them the option of doing that, so that the guests don't feel so left out or made to feel as though they are afterthoughts.
Ultimately, don't let your MIL's opinion influence what you do - if that is the way you want it, go right ahead - it's your day and your decision, not theirs
PS - I think you can have this thread moved to 2006, if you ask an admin...not sure though.