I love Kyoto, far more than Tokyo. Definitely worth a few days to explore if you are able to allocate some time there.
If you're going for summer I would recommend going as early as possible and doing Hong Kong first. I did Hong Kong over the first two days of June and I thought it was already uncomfortably humid. Shanghai and Tokyo (at least when I visited) weren't nearly as humid in that time frame. Because Hong Kong
Disneyland is also smaller and less stressful than the others, I'd also recommend doing it first since it is far less stressful than Shanghai and Tokyo, especially helpful if you're dealing with jetlag while you're there.
Yes. Many people could do it one day easily as there aren't as many attractions and the park itself just isn't as popular as Tokyo and Shanghai. That said, Hong Kong often has very short hours (and there's little to no downside to staying off property since there are no on-site perks, park opening time tends to be around 10:30, and commuting from the city is relatively painless) and some areas/attractions have special hours. I personally was there about a day and a quarter two years ago, and I would have liked a little more time, though ultimately I was able to see everything I wanted to. Many people probably would not need two full days for HKDL.
I would recommend two full days at SDL and four full days at TDR.