Just noticed this one lurking here. I don't have any answer for the Laird/Alligator replacement, other than the Alligator seems to have been adapted as a standard by the Rio Grande.

On the peaked roofs vs round roofs I have a speculation. I've never seen any definitive discussion on this variance but looking at photos gives me what seems to be a plausible explanation. Here goes:

The Baldwin Locomotive Works had a habit of taking plans for a standard gauge locomotive and shrinking the plans down to narrow gauge dimensions. You will notice that the earliest photos of 56 Class, and for that matter 60 Class locomotives have the round roof cabs and those cabs are very small when compared with later views of the same locomotives with the 4-panel peaked roof cabs.

I suspect that Baldwin's crew accommodations were more suited to the Stan Laurels on the crew board than the Oliver Hardy's. I also suspect that those Oliver Hardy's made their disappointment with the little cabs known.

So my speculation is that as the locomotives came into the Burham shops for overhauls the little cabs were replaced with substantially larger, or at least wider, 4-panel cabs. And why peaked roofs? Simple. It's easier and cheaper than building round roofs.

Make sense?