I honestly do no see the big deal with them going early. No one can control an aspect of your life unless you allow them to control it, so it may be annoying to hear their opinions on where to eat and what to do, but you can make a choice to ignore it and your own decisions.
I also don't understand when people talk about having their thunder stolen from them. Maybe I was just a reallly laid back bride (and laid back person), but even on my wedding day I didn't really have an all about me attitude as far as thinking that EVERYONE else should focus on me or adjust their arrangements to fit me. I was focused solely on me and my DH but I didn't expect anyone else (well, except for DH of course!) to be. To me that's just part of co-existing on this planet with other poeple. Other people will live their lives and do their own thing and make their own memories. Again, you are in control of your own thunder. You can either own the moment or you can allow yourself to get upset because others are making their own memories at the same time you are making yours.
FI and mom need to take a step back and R-E-L-A-X and adopt the same attitude you have, because once you go down that road of thinking someone is ruining your wedding then you will set your self up for failure because you will have that mindset going into it.
I think you have a great attitude about the whole thing and realize that this is YOUR special day regardless of what others do. No one can ruin this for you.