The release and resurrect abilities default to owner's territory. However, because Valley specifically says "take them to territory" you get to take your opponent's heroes that you release/rez. So yes, you're correct, you get all of them.
Because the second part of Valley is "You may do X to do Y and Z", I believe both Y and Z are mandatory if you do X (not 100% certain on that). So, your options are to banish Valley and either take and add all heroes to battle, or to let Valley be discarded normally and let all the heroes go back to their owners' territories.