[Rey_a]

In regards to backups, I would expect that backups, made in older versions of MobileSheets will restore to newer versions of MS without problem.
The other way around is a bit more tricky. If new features have been added in the recent version of MS that require a change in the database, they will also require a change in the backup file. Then the  older version of MS may not be able to 'understand' that new backup file when asked to restore it.

Bottom line :
 I expect it to work all the time when restoring an older backup file to a newer version of MobileSheets
 I expect it to work most of the time but not always when trying to restore a newer backup file to an older version of MS. 
If you want to be sure, keep MS at the same version on all devices/platforms you want to exchange backup files with. The bigger the difference in versions between MS on the devices, the higher the risk will become.

No experience with library sync, but I guess the same logic will apply

Rudy