I think @Sister_Momo_II has done a great job summing up the players' frustrations with this situation. And since you've mentioned in previous threads that gree is learning lessons from these situations and will endeavour to do better in the future, I hope this situation is one of them.
If you go back to the beginning of this thread, the op is from 3/8. We got absolutely zero response from gree, no acknowledgement that this was even an issue at all for 8 days when on 3/15 shisa finally says it's being looked into. And then somewhere around 3/20 it was finally fixed. So nearly 2 weeks after first being reported.
Because of this situation, I absolutely refuse to spend money on this game anymore. There's no trust that I will receive what I've paid for and no confidence I will be compensated should I not.
I don't know what the solution for compensating people like myself who lost items but can't prove it. I get that it's difficult since it was different for everyone, and only seemed to affect people playing on multiple servers. But here are some things I think Gree should have done to handle this better:
1) acknowledging issues much sooner
2) giving official word on how to work through them temporarily (exiting game before switching servers mostly seemed to work, but I only knew that because players here posted it and I'm lucky enough to read the forum)
3) explanation of how you'd like people to track missing items
4) making customer service aware of the issue and how they should properly respond to tickets
5) apologizing for the 100% gree's side problem
6) fixing the issue in a much more timely manner (the game was broken and didn't get fixed for 2 weeks)