I can certainly answer the last question. I have been to both
Disneyland and WDW on very slow days, and on VERY busy days during high season. The only times we did not enjoy the parks was if someone in the family was overtired, burned out, or had a bad attitude. Being with someone with a bad attitude is the worst, believe me.

I have done it a few times with relatives who have nothing positive to say, and we won't repeat that mistake again.
If you measure happiness by the number of rides you do in a day, then it can be very frustrating if lines are 60-90 minute waits on standby. On the other hand, we have been there during slow periods when we rode so many rides so quickly that we wore ourselves out, and got overtired early in the day. There are nice restaurants, hangouts, shops, niches, shows and street entertainment, plus the resort amenities and Downtown Disney. If you are into turbo-touring or commando-style approach to rides, you run the risk of burning your party out - especially if you have kids. You need to take time to appreciate the Show.
They do close the gates at Disneyland on very rare occasions if the crowds are too heavy, but that is very rare. On very busy days, they add shows. Now that DCA is open and generally uncrowded, you have that option as well.