I've always thought he was most formidable at WW and I still do. However, when I completed my analysis to support my all time rankings a few years ago, I was shocked by how few fights he contested at WW and how many he fought at MW. His record at WW is around 39-0, including 6-0 in world title fights. His record at MW is around 104-17-6 and at one point was 66-1-2, with the only loss to LaMotta avenged. He went 8-5 in world title fights, but most of those losses were suffered way past his prime. So, I consider that he was a better WW than MW, though fought more often at MW, even before vacating the WW crown.
Welterweight was certainly his best weight tho he might well beat anyone at 160 at his best. That's why he's considered by many to be the greatest.
I guess so, although you could make an argument that despite the kinda quick wear and tear fighting stronger dudes at middleweight,he was mainly more of a middleweight fighter,unlike Duran who spent a first half of his career around lightweight/super lightweight for non title matches.
True, although keep in mind that he was weighing in on the day of the fight. Also, he weighed in the low 150s for several middleweight fights. Day before weigh in like today, he would remain at welterweight for a lot longer.