What if Marciano hadn't retired?

[ICB]

he got knocked down plenty of times

I don't think so, Lyle - he was only knocked down twice in his entire career (Walcott and Moore). The Walcott knockdown was only a flash knockdown as he got straight back up and steamed into Jersey Joe.

I can't think of any other heavyweight champion who was actually knocked down less times than Rocky.

kind of skirting the rules here but other than to Tyson did Spinks ever go down?

He was knocked down by Dwight Muhammad Qawi i don't believe anyone else floored him other than Qawi and Tyson.