Very well put, agree 92.6%
It's a bit of a quibble to say cost of replacement isn't necessarily relevant. Obviously, it's as good a proxy as we have to try to get a handle on the value of what you destroy. Let's put it this way: if a kaleun sank the HMS Hood, would it really matter what he did the rest of his career? Of course not.
Personally, I wouldn't engage one of these wandering DD groups any more than I engage coastal merchants. I don't go after anything less valuable than a C2 unless I'm on the last leg of the trip home.
I'm of two minds on targeting the convoy escort thing for the reason you gave: you usually don't know what else is in there unless you've shadowed the thing for a long time. But at the same time, I'm usually greedy and plan on reloading and taking another bite at the apple in a few hours, so taking out the border collie is very tempting.