I've got an old Raleigh with a 531 frame which I rode half way across the country before the back end imploded. I'm looking at the derailleur hanger and I see that it is bent almost to the cassette. Question: if the hanger is bent back out so that it's straight again will it be strong enough to support a new derailleur or is it doomed to be a single speed from now on?

I'm not sure about your case. I know slightly bent mtb hangers can be straightened and they sell a tool to make sure it is aligned with the frame properly. I bought a new hanger for mine and the instructions say to still use the tool to make sure the new one is aligned properly. I'll probably try to wing it because the park tool is $80 if I remember correctly.

It will probably be fine if it's straightened carefully. Take it to a bike shop with an alignment tool and see if they can help you with it. Steel is resilient stuff-

I have an alignment tool but I seldom use it. I think you can eyeball it back pretty close. I just tighten a properly threaded bolt onto the slot and bend it back. As long as you go working it back and forth a few dozen times it will remain strong and solid. Steel is real.

..DON'T go working it...

Why did it "implode"? Was the hanger the only part that imploded? If there was nothing unusual happening when the hanger bent, it may be too weak to maintain position if reset.

haha...the chain broke and got stuck in the derailleur (as near as I can tell) which flipped upside down and bent in half and then the hanger bent too. a very discouraging thing to have happen far from home. If anyone has any knowledge on what else could have been the cause please let me know.

Clipping a lawn gnome while powersliding on dew-covered grass will cause similar damage.