Different types of ～そう
I think it's best to consider adjective + そう and verb + そう separately, because they have slightly different meanings. In particular, with verbs, そう has the sense of something close to happening or about to happen in the near future, but with adjectives... not really.
It looks like it's about to rain. (near future)
She waved happily. (not the future)
(I haven't eaten that food yet, but) it looks tasty. (kind of maybe the future?)
Negative form of ～そう with verbs
Other answers have already mentioned that the さ in 降りなさそう is not the officially correct form, and I found an authoritative-looking answer on Oshiete! goo which agrees.
I believe that
all mean the same thing, and I found a link for that one too.
Difference between ～そうにない and ～なそう (verbs only)
「雨が降らなそう」is the normal negative, which just means "it doesn't look like it's going to rain. I think ～そうにない (and そうもない and そうにもない) not only add emphasis, but also imply "I wish it would happen, but it looks hopeless." Your earlier example 「僕にもわかりそうにない」, "It looks like there's no way I'm going to be able to understand it either" also fits this pattern. However, it's more difficult to back this idea up. I did find a link where someone wrote that 「授業に遅れそうにない」 seemed unnatural, but they couldn't explain why.