[EricH]

I think it's important to note that the plywood does not have to enter the corners at all (R21B, bottom right image). I would say that the joint is OK.

For the metal segment, that's a "no"--there's nothing in the bumper rules allowing it (and bumper rules are the one part of the manual where if it isn't in the rules, it isn't legal). HOWEVER, if you were to design that as part of your attachment system, that would be fine, I think--but it would count against bumper weight.