but 6.5.9 now keels over and over with a segfault after about a minute each time
A segfault means that DSLstats is attempting to read (or failing to write to) something which is no longer there (in memory).
This infers that segments of the storage drive has become corrupt. The storage drive (memory drive) can be a hard drive, SDcard, USB memory stick or even CD/DVD depending upon the type of memory medium DSLstats is running from.
Basically - your storage medium is duff - most likely caused by the power cut if the operating system was busy at the time and something was attempting to write to disk at the time power went off. Depending upon the type of storage your operating system may be able to run a repair or declare segments as unusable. However. take it as a warning telling you to get new drive
asap, before the whole drive fails.
To be more specific the error you are seeing means that part of the drive holding the prog DSLstats is now corrupt and therefore it can no longer run properly OR DSLstats is trying to read data stored on parts of the drive which are now corrupt OR the drive is now so corrupt that DSLstats cant record (write) any new data.
For more info see
Unreadable File Record Segments---------------
PS I did happen to see your reply to one of my moderators the other night, but I was not in a position to type at that time.
FYI He was trying to be helpful - for all he knew and from the info in your first post it could have been part of the drive that held the OS which was corrupt. As he said, power cuts are well known to cause failure of the memory drives particularly on R-Pi's which do not always take kindly to not being shut down correctly. It wasn't clear that you were running from a USB memory stick. If everything else on the memory stick appears to be working OK then I'd guess the bad segments are where DSLstats was stored on the drive so its caused the prog to become corrupt, so you will need to redownload it. It is time though to get a new drive because you dont know what other segments are damaged too or if other parts of the drive may not also fail in the near future.