Yeah, just after you get off the flight and head for connections, everyone has to go through a BP checkpoint. When I do this, my PYOBP fails to validate, and I'm sent to another desk right next to it. Usually no queue, no hassle. They check my PP, validate it, and you can go through the checkpoint straight into T5, it's pretty easy.

I assume if you check in at desks, this is done at check-in and thus no need to do at T5.

Connecting domestic to international at T5 doesn't require the schlep through the FCC you go through on returning, there's no security checks at all, so as soon as you're validated you're in the terminal straight away.