If Floyd hadn't retired, which of these Welters would he have fought?

In Arums own words he stated that he would have fought Margo for 8mil. Meaning he agreed to that fight in the 3-5 fight package that Arum was presenting him. He also asked for 10mil to fight Cotto. I don't see how those wasn't a reasonable asking prices when looking at the money that these fighters are making today. Arum only offered him 7mil which I think people can see as a low ball offer today. Then there is the fact that Mayweather called out Mosley in 1999 when he was even more dangerous and again in 2006. Mayweather also moved all the way up to 147 in order to be able to fight Mosley and DLH in big money fights.

Why would he fight Paul Williams during that time frame? What made Paul Williams the most worthy opponent to fight PBF at that time?

Same stuff every day.

To answer the question he would have fought anyone that presented him the best money deal.