Very true. But assuming we’re still talking about doing this on the gateway: backing up and forwarding are two completely different things too. 
I very much agree.
That’s the same waste of time, I feel.
Only if one has the (OTAA) session keys that were applicable at the time the packets were received, and if one either (intelligently) brute forces the MSB 16 bits of the frame counters or keeps track of the frame counters too. Like I wrote that’s not as easy as one may think, I feel.