Thank you, that makes sense. I suppose my only remaining question from the previous post is: it seems you would need to also do something very much like the “SPV Proof” described in the previous Sidechains paper, except expressed somehow (??) as a condition in the “WE” scheme used in the PIPEv2 scheme, is that correct?
I won’t pretend I understand the maths and I assume that somehow the encrypted key in the PIPEv2 is… somehow invisibly created by some kind of multiparty setup, maybe? 
There are other options that one can use, like atomic swaps that can be done with any amount.
That is fine for a lot of day-to-day operations, but would be more akin to offchain-onchain swaps in Lightning than true unilateral exit in Lightning; without true exit, nobody would trust the swaps.