Iride01 

No, two steps required..... stop timer and reset it. But you correctly stated it at the end.
I wouldn't go so far as to say "deeply stupid" or even "stupid". It's just how they have done it in all their edge series and other sport and activity devices that I'm aware of. If you read on Garmin's own forums and elsewhere there are some that aren't even starting their timer and wondering why they don't have any data, or others as I've done myself, stop the timer for a lunch break and forget to start it again. I've often wished that their edges and other activity devices would do like their marine chart plotters and handhelds I own and record any time the unit is on. Use the stopping and starting of the timer to just mark the part you actually consider the start and end of your ride. That to me would let you always have data to pull out parts you may have forgotten to start the timer with. But, they don't. I don't know if anyone else does either. So they all must be "deeply stupid". Though it's really just a programming choice of the first development team. Just guessing it's partly based on the days when memory was expensive. My devices that log continuously do overwrite if I fail to save the data off.
Well they do, it is called "reset". Although they try to force it on you more often with the later edges and call it "save", it really is doing the same as the "reset" button. Many don't realize on their later edges that they can ignore the "save" message and go to other screens, turn off their device, or just simply start the timer again to continue the ride. Same exact stuff as the older devices with "reset" Only difference is new device use a prompt on the screen, older devices have a button. Sure there are software code differences, but the user actions are really much the same.

The way you describe doing your timer on your multi day trip should have worked. The problem being whatever was the "white-screen" you mentioned.... low on battery or software exception, There may well be some limits in the code that are being exceeded and throw an exception that is not handled. But being proprietary software, it's up the developers to do that, but the developers are probably doing other higher priorities..

I've ran my edge500 for up to four days before and quite a few stop/starts of the timer too, but never lost data. However I seldom use course navigation or workouts so maybe something in there alters some of the reliability. But I don't think it's the 800 vs 500. You can find posts in all the individual device sub-forums on Garmin's site that express similar frustrations with lost data. If it can't be traced to not starting the timer, then it's similar to your experience.

So that's why I and I'd guess for the same reason others recommend saving your data in smaller segments. Just helps ensure you'll get something. And as @TimothyH mentioned you can combine multiple activities later. If you need to know your total event time at any one moment during a ride, then would a timer on your phone or another device be a safer choice in light of how the Garmin may let you down? My phone actually has a cycling app on it that records everywhere it goes whether I'm cycling or not. It's been pretty accurate for the few times I've pulled the data. Since I'm not using it specifically to record an activity, I have to go into the data and pull out what I want.