E.G in the original situation it's easy to see that A turning his cards over is an indication he believes the betting is done. But if in the revised situation above - the river is another brick and the bet he is facing is far far greater than £20 - then he could easily say "of course i'm not calling it, why do you think I turned my cards over? I'm passing." And B is now robbed of money perhaps rightfully his.
Let me see if I can complicate this a little further

. I think in the original version of the scenario it does seem as though Player A believed player B was all in and he was turning his hand over because he thought the action was over. HOWEVER, the dealer, player B (especially) AND player A all watched the dealer return some chips to player B. So why doesn't player B say "hey, what's this? I am all in". Why doesn't player A say "hey why does he get that back? I call" (and of course, what the heck is the dealer doing in the first place).
So, in your second scenario there would be a lot more chips floating around to bring attention to the situation and I don't think it would actually occur. If there were that many chips pushed back to player B and player A turns his hand then he is actually in trouble for exposing his hand and would be stuck having to decide what he would do when the original river comes out. He would have to check and see what B does and then decide if he was beat, then get a penalty.
And if player A does know there are chips behind player B and exposes his hand, not only is exposing but he is technically soft playing player B since player A believes he has the nuts at the turn and IF the river is a brick he is letting player B off the hook for the last 20 chips (or whatever amount).
I guess my point is, the error that caused the problem started with the dealer and compounded by both player B. And if there where more than 20 some chips I would think Player A would notice (should have anyway) and you wouldn't have the second scenario you ask about.