We just got back from WDW with our now 4 year old, and we were worried about many of the exact same things that you are talking about. Thankfully, however, everything turned out fine.
Small World is the ride that he talked about the most - in fact in DS-speak, the phrase 'small world' actually means WDW. But we explained to him for about a month in advance of the trip, on various occasions, how IASW was going to be closed on our trip. I was worried that he would be crushed, but he rolled with it very very well, and actually picked up a new favorite ride - Buzz Lightyear.
If it's really going to be years in between trips for you, and IASW is one of your all time favorites, then sure - bump the trip back until May.
By the way, we stayed in one of the hotels that people complain about the motion activated air conditioning - Beach Club Villas, and we didn't have any problems at all with it.
If you like to stay in the types of on-site properties that cost $600 a night, perhaps you should consider buying a small number of points via re-sale in the Disney Vacation Club. It actually makes going to WDW much easier to handle financially for us. You could buy 100 points on a resale contract, and only use the points every third year. That way, you use last year's points, this year's points and next year's points all at once, and it would pay for itself over taking trips, however infrequently, at $600/night.
I sympathize with your concerns, just wanted to tell you how well things worked out for us when we were extremely worried about a lot of the same things too.