@LIssie, I have some issues with your proposed plan... 😎
For context, of the "islands" you are considering, I've been to Sicily, Sardinia, Malta and will be going to Corsica in September 2024 (so have done a lot of planning for Corsica already). Can't help you with Cyprus or Crete (yet).
I"ve been told he wants the trip under 2 months. But we don't want to
drop any islands LOL.
There's the problem (well, one of your problems): Six "islands" plus Istanbul and Milan. "Under two months". Not a wise set of criteria.
One suggestion: stop thinking of (most of) these places as "islands." Yes, they are surrounded by the sea, and technically they are. But some of them are pretty large (Sicily, for example, is roughly the size of Belgium) and most of them lack perfect road systems, so it can take a while getting around on narrow, twisty roads along coastlines or through mountains. They each seem to have fairly distinct cultures and complex histories, making them more like separate small-ish countries in some ways. They're also not "single places" - some of them are regions.
For the places on your list that I've been to, your proposed time there seems awfully short - all of those places are worth 2-3 times the number of days you're considering.
To address a few of your specific questions...
September/October is the ideal time for thee places; the weather should generally be nice, the sea still warm enough for swimming (though by the end of October not for very long!). I suspect November would not be as enjoyable.
Corsica (there are direct flights from Milan)...
Check flight options carefully. I've been looking very intently at flights to Corsica, most are highly seasonal, with many flights dwindling or ending completely late summer/early September. Be careful if building a plan around that. We're going for two weeks and it's painful trying to squeeze in what I want in just 14 days.
Sardinia 7 days (short ferry hop) probably hire cars on each island
You probably can't take a car from one island to the other (certainly not for a one-way car rental). We spent almost 3 weeks on Sardinia and wished we had had another two weeks there. It's large and surprisingly diverse.
The two islands do have some things in common (those are all good things!) but very different histories and distinct cultures. Ever since we went to Sardinia we have been longing to visit it's French neighbor.
Malta (maybe fly) 5 day Probably not hire a car...
We did two weeks on Malta & Gozo (and we could have spent more). If you like Italy, you'll love Malta; it's like Italy only even more Italian (in fact, it's a lot like Sicily, but goes "beyond Sicily"). Conversely, if you really dislike Italy, you might want to drop Malta. Malta is a strange mix of Italy/Sicily/North Africa and the Old British Empire. Malta's history is fascinating, but most visitors only go as cruise-ship day-trippers. Without a car, you'll be stuck in the capital city with them all, missing all the good stuff that's beyond day-tripper range. Gozo (the second island) is awesome (some people say Gozo is the best part of Malta, like "old Malta" before the advent of cruise ship crowds and mass-tourism). We loved both.
IMHO you just have way too many places and not nearly enough time in them (don't get me started on just 5 days in Istanbul...).
If you really dislike Italy, drop Malta, drop Sicily, and drop Sardinia. Then spread your days across the other places (in Turkey, go to Istanbul and beyond). But no matter what you do, slow down, the places on your list are worth a lot more time than you're giving them.
Hope that helps a bit. Good luck.