{{{{{{{{{{{{{{{HUGS}}}}}}}}}}}}}}}}} to you for your painful back, I know what is like, and its not very nice at all.
You could do that, because when the am nap is extended, and he knows how to do it, he should follow suit with extending his pm naps.
He is likely to alter with age, because you are teaching him how to transition from one sleep cycle to the next, that will hit him and he will think, oh I know how to do it now.
I don't hear a peep from my DD in the morning (1hr30mins) and afternoon nap (2hrs). She went suddenly to that, at about 5 months.
I was going to suggest taking the side rail off, but I read further on that he is turning upside down in his cot. Yes, I think it is safe, because my DD does this all the while, and I have never been bothered by it.
Extending naps is nothing to do with the shout and the mantra cry, it is what some babies do when they are transitioning sleep cycles.