With no experience directly with the second material you mentioned, i could say that the speay marerial is very easy to apply and it flows fairly well filling in cracks and gaps.
yes, it works real well for that
I think you can try it.
no once it dries on your skin it hard to clean, try using rubber gloves when applying
I would say yes
I did use in my shingle on roof that did not work as it say in the can
yes it will adhere to dry silicone caulking
Yes it should properly seal a leaking gutter - I assume your leak is at or near a connection? The product is black and messy so be careful when spraying it and only use it in the inside of the gutter as any product on the outside will make your gutter look very ugly.
depending on many things, i t would work on sealing heavy plastic. but you should use several coats not just one coat.