I reply only to items 2, 3 and 5, because I think the answers already provided to items 1 and 4 are quite satisfactory.
2. That is in my opinion a major plot hole: Greenleaf seems a very clever guy, so he had no reason to conceive such a difficult fake evidence as a synopsis of the novel typed on Kane's typewriter, that would be very time-consuming and dangerous to be discovered while doing, without mentioning that he had to wait until Mallory had dictated the book's end to write the synopsis. Moreover, the novel was supposed to be kept secret by Mallory, so Kane could not have known that his plot was being plagiarized. Greenleaf could more simply have claimed to Columbo that he, Kane and Mallory had a conversation about a plot and Kane later knew that Mallory was writing a novel (this fact was no secret) and suspected Mallory and Greenleaf to have stolen his idea; a claim that Columbo could not disprove, there being only reported conversations.
3. Greenleaf would have had great difficulty to replicate a key he didn't have: the locksmith would have suspected it was an illegal request, and that would have been a witness against him. Moreover, as previously explained in other posts, he had no reason to duplicate the second key. Greenleaf doesn't seem over-anxious, and he would surely have remembered how Kane explained to have entered through the open door, so there was no reason to plant a fake evidence of the second key. It isn't difficult to suspect a trap by Columbo with his insistence in finding the second key; it's strange that Greenleaf didn't recognize that.
5. Given what is stated above, I agree that this episode is one of the less plausible.
Anyway, I consider Jack Cassidy one of the most brilliant players as the offender in Columbo's series.