sonny having more holes than swiss cheese and the don fertilizing his garden.. who would have been the don if mike was found and killed?

There might have been an internecine struggle among the family's capos. However, Barzini and the other families would have used their influence to pick a don.

Your time frame isn't clear. Since your question puts the time frame after Vito's death, I assume you mean if Michael had been killed on his way to that meeting that Tessio set up with Barzini. Then Tessio would have taken over and would have been a satellite of Barzini.

If you mean that Michael would have been "found and killed" in Sicily instead of Appolonia, Vito would still have been alive. He would have had to govern as long as he could, and would have divided the Corleone territory among Tess and Clem.

Without giving Fredo and Carlo anything?

Well if Sonny is dead then everyone still would have known Carlo assisted so even without a Michael to turn things over to Carlo is not going to get anything from Vito and friends except a quick trip to the afterlife. Vito may have still wished his friends to wait on that until after he's gone but he's not a forgiving person.

Fredo was incapable of standing up for himself or family interests in Vegas so even without a Michael to chastise him, I don't see Vito giving Fredo any real authority or power in the business.

Considering the types of people in Vito's world, the best way to protect Fredo after Vito was gone may have been NOT to give Fredo anything anyone wanted.

Why wouldn't Vito have made Hagen the successor Don?

I don't think Hagen was quite leadership material (no knock on Hagen by definition most people aren't) and just as importantly I don't think even if Hagen had been well suited for the job that he would have been accepted by the other Family members, not to mention the other Dons.

Furthermore, no one who wasn't Italian could become a Mafia don (except on an informal temporary basis, like Michael did with Tom in GFII). Non-Italians can be consiglieres and associates of Mafia families (like "Jimmy The Gent" Burke was to the Lucchese family in real life, for example), but not made members and certainly not dons.