Hi Monorail - I excerpted some of your post to chime in on a couple of things based on our experience last August....
We did the Family Bedroom with 2 adults and 3 kids -- 13, 10 and 8 at the time. Yep, we knew it was crazy, but we wanted to try it and we were all up for the "togetherness challenge"!

They are all girls, and my eldest is very small for her age (as in, still well under 5' tall), and the rest of us are on the shorter side, as well. Like you said, that was a trial-and-error trip for us, as previously we have travelled coach. It worked out fine and we are doing the family bedroom again this upcoming July, but we all agree this will be the last time. If we do AT again it will probably be a trio of roomettes or maybe a bedroom and roomette (in either case DH will probably claim a roomette to himself so he can get away from all the estrogen for awhile!

).
I will admit DH & I did not get a normal night's sleep, but the kids slept like rocks. I slept with my youngest on the bottom larger/longer bunk, my older girls each had one of the shorter side bunks, and DH had the longer top bunk. We were all able to stretch fully out (again, none of us are tall) and it worked fine for us. However, I should add that we did not spend much awake time in there. Between the happy hour, dinner, showers, and visits to the lounge car, there was hardly any time when all 5 of us were in there, so we did not have an issue with feeling claustrophobic. There's no question it is tiny, but it worked OK for us.
Showers -- we tried them and they were adequate. My girls are used to showering at night and so we tried it. We kept it quick and didn't shampoo hair. We wore our flip flops to avoid bare feet in there. It was definitely tight quarters but it is doable. They do provide stacks of towels & washcloths, and there's a bucket with travel sized bars of soap and packets of shampoo if you don't bring your own.
We did the priority unloading on the way down and were happy we did -- we were anxious to be on our way and we were about 5th or 6th off, and it had us all in a great, vacation mood to be headed right out like that. It was a nice splurge but definitely not a necessity. We did not do priority on the way back as we were not in a hurry to get home! But we were there almost 2 hours waiting until finally our van came off, and it was drizzling so we didn't go outside for fresh air or playground or anything. I was so distracted waiting to hear our number that I couldn't settle in to read or relax. The more time that went by, the more I was thinking "we must be next...we MUST be next!" and I just couldn't concentrate on anything else (maybe I have a short attention span, lol!). That was killer because we knew we were facing a long drive and would have been nice to get on our way (in retrospect, waiting a bit when you are essentially at your destination, and doing priority to get started on the long ride ahead of you, would probably have been smarter, but we were so excited to get to WDW that we did it that way

). It's all luck of the draw, I think, on when your car comes off, so it's just a gamble on whether you will be waiting or not. We haven't decided what to do this year WRT priority unloading.
Thinkin' beachy thoughts for you!