I feel your pain on the planning issue. My DD17 is a senior this year and last year's spring vacation was spent on college visits as was most of this past summers (4-5 weekends total with one week at the beach). In July my DH and I decided one last trip to WDW as a family was in order. It's not that she won't come in the future...but it will be harder as her vacations will not jive with her younger sisters....and of course the collegiate returnees want to be at home during their breaks to see their friends - NOT with their parents.
So our July plan turned into a surprise Thanksgiving trip - with 1.5 and 3.5 days off for our two high schoolers. We left on 11/16 and just got back today. But....when we announced our "surprise" plans to our older DD's....the news was not so well received by our senior. We didn't realize in July that she would be in charge of the school spirit day which was on Wednesday before Thanksgiving break. How in the world were we to know that? I'm willing to bet that the administrators time these things so kids won't want to take days off from school! We've never taken our kids out of school before.....
So she was seriously pouting for two days and then we gave her an option - fly home early (only because we found an $80 fare on Delta), stay with Mom's 50+ yr old cousin, but no use of car except for schoolm (it's a 25 min drive), Thanksgiving with mom's relatives on Cape Cod and stick to scripted schedule. She took the deal. She was happy enough with the plan and we didn't have to forfeit the whole trip or have her sulking around Disney for 4 days.
And we had similar planning obstacles as yourselves. My sister who is single is having her first child - probably with complications - perhaps as early as next week although she's technically not due till December 13th. I knew we couldn't go over Christmas and we have a trip to Hilton Head planned for my DH and his golf buddy in April. This coming summer is reserved for a family reunion trip to celebrate my DH's parent's 50th wedding anniversary. (There'll be 20 of us - oh joy!) And then of course there is college orientation and the packing for college, etc. So this was it for Disney for a couple of years.
Anyway...back to your planning problem. We've gone in July (13-21?) and the week before Labor Day. Both were hot and rainy. But in all honesty.....after this frigid week at WDW....I'd take July or August in a heartbeat. The crowds were manageable in August and you could count on one shower a day - a bit of thunder and lightening and then it was over. ?We did La Nouba, Disney Quest, shopped, saw a movie, etc. We were still on vacation and just adjusted.
July was a bit more humid and a bit more crowded. But we're used to the April/Easter vacation body count and found July and August "easy". We just spent more time by the pool and less time at the parks. This was of course in 2001 and 2003 before the wave of hurricanes that swept thru Central Florida around Labor Day.
And this week was no piece of cake with the cold temps and the crowds. I found the lines to be tougher to endure when you are shivering. If the temps had been higher perhaps I'd be singing the virtues of taking the kids out for the few days before Thanksgiving. But you have to arrive in the parks by 11/15-16 to make it worthwhile. By Sunday 11/19 the MK was busting and it was twice as hard to get on a ride without a fastpass as it had been on the previous Friday. Would I go at Thanksgiving again - probably - someday. Am I going to reserve next year - no way. If I'm going to go to FL I want it to be warm at least.
We went to WDW once in February and I said "never again". I thought we could squeak a week in November and it would be warm enough - but it wasn't. I used to live in FL and I remember December vacations home from college lying in a makeshift barrier between lawn chairs covered in towels determined to get a few freckles before heading back to school. I was freezing..... How could I have forgotten that?
As another poster said....it's just all about adjusting your expecations and you'll have a wonderful time no matter when you go.