He can't finish his career on a loss, or even a KO, so no I don't think fighting against Mayweather would be the best choice for him for his last match before his retirement. He must find another opponent to close his career after this fight.
Manny Pacquaio is not weak like that since you are implying that.
He did lose in their first match but I don't think Pacquiao will just end it like there. It is not that he lose or win in his last match it is how he wants it since Pacquiao really wants to fight Mayweather. They fought in 2015, did he knocked down Manny? No, it is a unanimous decision though. I think Manny can win against Mayweather, he can. Though they are saying that MW is scared.
https://www.youtube.com/watch?v=AND4RihvQ_AUnfortunately, a WIN of Floyd is a WIN, regardless of what fashion, its still a WIN. We just don't like the way Floyd fight because he hit, he move, he hit, he move, he doesn't wan't to engage in a slugfest in other words he is boring but that kind of style gave him his 0 loss and its legal. If Senator Manny wan't a piece of Floyd again, he needs to knock him down or win via TKO. If its through decision again, its going to be Floyd.