First, let me just state that terms like uncle and cousin have broad colloquial meanings. Uncle can be any older male relative or friend of the family, cousin can be any relative outside of your immediate family, etc. The terms do have specific genealogical meanings, too, but you don't have to stick to those, especially in complicated situations like the ones you present. I've tried to go through the genealogical uses for each situation.



The individual in question is your spouse's uncle's first cousin-in-law, and therefore your spouse's first cousin-in-law once removed. Technically, you're related to this person by two marriages, but adding the "in-law" again is rather redundant, so I would say that you could just copy your spouse's relationship and call him your first cousin-in-law once removed. A relationship like that can be shortened for ease of conversation to cousin-in-law or just cousin, depending on your preference.



I suppose you could call him your step-uncle. It sounds a bit odd to me, but it's accurate. Just calling him uncle would also be acceptable, especially if your step-father has adopted you.



I'm not sure what you mean by "you are related to that last child"; related how? The last (fourth) child is the half-sibling of the third child (linked by their common father), and has no blood relation to the other two children. If you're related to the fourth child on his mother's side, you have no blood relation to the third child. If you're related on his father's side, you're related to the third child in the same way.