Second scenario is highly unlikely. Alvarez is a fairly measured fighter with a moderate punch put output, even compared to +35 year old Pacquiao. Pac still has way more options than Canelo.
Getting dropped, on the other hand is a distinct possibility, especially if Pacquiao doesn't reduce his output a little bit and mix up his tactics to avoid patterns. The weigh in photos reveal the height & size difference much more starkly than in street clothes.
Oddly enough, if you go strictly on the form of the most recent fights, you could argue that Pacquiao might have the edge. He was way more impressive vs. Bradley, Rios & Algieri than Mayweather was against Alvarez & Maidana IMO.
Of course, there is more to it. When both fighters have lost some athletic brilliance, the edge usually goes the the one with the stronger technical foundation, who does not have to work as hard, and depends less on special physical powers. That fighter is definitely Mayweather.
Also, Pacquiao is not going to enter the ring weighing 165 lbs like Maidan, nor be able to treat Mayweather's punches with that kind of contempt.
Most importantly, Mayweather knows if he loses, he'll never hear the end of it. If there was any complacency or lack of edge in the preparations for his last few fights, it certainly won't be the case for this one. .
So unless somebody shows up suddenly as shot as Sergio Martinez, it should be fairly comparable to Mayweather-Cotto, a reasonably well contest competitive bout but with Floys a fairly clear winner.