Yay, Israeli elections!
Of course, this scenario (which to me is a desirable one, other than the ascension of Yair 'No Backbone' Lapid) would be nigh-impossible to achieve- I don't think Gantz is eligible to run, and Hatnuah is now a spent political force. Also, Labour kept Herzog
Still, since this is pretty much an alternative universe (maybe the cooldown period for generals didn't pass as a law), I like it! The first scenario you suggested
could be likely, though with Netanyahu leading Likud, I don't see the second happening. He and Lapid are two bloated egos. If Netanyahu were to be replaced at the head of Likud, I could see that party joining an already existing government though.
I think that the likeliest scenario would be a YA-HaAihod-Hatnuah-Kulanu-Labour one, since Lapid wouldn't want to appear like he has a left wing ideology (or any ideology, really). Maybe if Meretz are pragmatic in this universe they could join for a bigger government, not dependant on just one party.
I tried to craft a scenario where Gantz leads a government, but that would require a weird mix between Haredi parties, left-wing politics and Kahlon. Not impossible, but not likely. Meanwhile, Netanyahu is probably blocked out of forming a government because of a very weak right and the rivalry with Kahlon. So Lapid, unfortunately, is almost certain to be the next PM, but he wouldn't be a terribly strong one, probably having to give out many ministries and with almost every party able to make demands. He'd probably strive to get a Netanyahu-less Likus in opposition, which would leave his left-wing partners unable to make threats lest they're replaced by a right wing party in government.
This is such a great scenario with so much analysis to do, though- well done!