I'll be honest, I've never flown into the Orlando airport and I've never taken the Disney Magical Express but I know where the airport is and I know how airport shuttles run in other cities with major airports, with that said, I don't see you making it there in time. I'm 99% positive you won't. Their is just too much out of your control here.
First of all you have to make sure your flight leaves on time to even have a chance of arriving on time. Secondly, depending on where you're sitting on the plane will be the difference on how fast you can get off of it, this is any where from 5-15min. Thirdly, you have to walk to the DME bus, get on and it has to leave. I've always been told this process takes anywhere from 30-60 minutes by itself, just depends on how many people its picking up. Fourth, it has to make the 20-30min drive to WDW and finally, once on Disney property it may not go straight to your resort, you may be the 1st stop, the 3rd or the 5th.
Short answer, I'd move the time back absolutely.
BUT on the second thought, idk how close you are to your MIL but since she planned this unless you really wanna eat at the wave don't worry about it.
PS, third thought, if you don't make it to your reservation you get charged $10 per person in the party.