Nothing's changed. The point of a bounty is to get attention not an answer. This doesn't necessarily guarantee an answer. Our policies on that haven't changed. If you have the rep, and want the answer you'll just have to keep posting (bigger) bounties until you get it, and see if the extra attention will help.
A bounty is best used as a last ditch effort to ...
I understand all comments warning that this would corrupt the SE community, but still I think this could be done in a "stealth" way, e.g.: there's already a feature that allows you to put a bounty to a question that has been unanswered; this feature works by subtracting reputation points from your own account to add to the question, but what if there was a ...
The ability to resurrect an expired bounty by adding more rep to it.
That's already possible, and you're giving the example yourself. A 50-point bounty can be doubled three times, and you can post infinitely many 500-point bounties (as long as you have the reputation, of course).
Getting bounty returned if there is zero response (no comments, no ...
This has been status-completed 6-8 years ago:
To see your own ones (faster than auditing your reputation), just follow this link:
The data is available in SEDE; for example, this query lists all 4688 bounties on this site. Since a question might be bountied multiple times, there are duplicates in the list, but I'm not sure if you mind.
Feel free to fork the query to suit it to your own needs.