Of course he doesn't know for sure whether collusion happened. That's why there are investigations going on to find out if that happened, which they wouldn't need to carry out if they already knew the answer. When you have a foreign entity meddle in an election in favor of a particular candidate, and that candidate's campaign had multiple contacts with representatives of that foreign entity, you need to have an investigation.
Yes, but why leak if there is an ongoing investigation? The only "legitimate" reason is that you think that the important information would otherwise be "covered-up" or if it really urgent (for example if Trump was about to sign a new tax law, it would be reasonable to leak his tax-returns if those were somehow conflicting with this law, etc). So why not wait until the investigation is over. Then, if it looks like a cover-up or somehow is corrupt/wrong, leak. Then!
Otherwise, some people might think, that the only reason of leaking is to damage the president 
How is this leaking, or are you referring to earlier incidents? Isn't he free to give an answer as a former CIA director if asked before Congress?
