For the boston crab, I can’t throw you the stone because that was my first port of call as well. Because really the “do what you want with the other hand” pretty much open up pretty much what you want to do.
The only the thing that goes against it is that, as far as I can tell, it s not described in any manuals I know off.
A few years I have tried it in 15th cent armour (German and Italian) and it does pin but the folds and the cuirass just prevent the hyper flections and the poleines lock up before the stress. Pretty much the same way that ude gatame do not really work when you wear an arm harness.
But anyway you can get an ankle twits/lock instead of the crab proper
That being said I should get it done by someone who can do it properly.
In any case, it will definitely work with transitional armour, mail, mail +coat of plate or 16th cent armour.
As far as the way of turning
for what I tried I think it does work well and fit the text, once you the turn has been you can get a good enough control on leg the ankle you scooped up. Calling that bit a lock may be overly generous but it is enough to stab him in peace or to move on to something else.