We're doing an Illuminations reception too - Ours will be on April 24th. Our ceremony is at 5 and reception is from 6 to 11.
We're doing the busses and the $12.50 viewing fee. I *THINK* though that if they all meet at the IG and walk in with a Disney escort you can still do the viewing fee only. I believe it's just the Bride and Groom that can't use a regular entrance. We have to have busses because we're getting married at the WP, but if you're at the gazebo and want everyone to walk or take the boat to the IG, I don't see why that wouldn't work. Then you and DF can take other transportation - Limo or towncar I think are the options.
Oh, as far as the food goes. The only meal option at French Island is a buffet because there's no kitchen nearby. The choices are really nice though - you use the EPCOT catering department and they have a huge menu and are very open to ideas beyond that. We did our tasting with them in September and my parents who have been to Disney many times were so impressed they talked about that tasting the rest of the week. There are a few food limitations because you're outdoors - no mouses for the cake, no ice cream, no frozen drinks, nothing that they think won't keep outside. We were able to do all the food we wanted, just had to switch from chocolate mouse filling in our cake to chocolate ganache filling, which wasn't a big deal to me.
If you have any other questions, I'm happy to help!