db2-Backup/Restore is only possible between operating-systems when the CPUs are compatible (as regards endian-ness) and encoding. So if you are running AIX on Power hardware, and running RHEL on X64 then you cannot backup/restore between them. You can use instead db2move or other similar tool (i prefer parallel-export and parallel-load scripting).
You don't need to specify the compression library unless db2 can't find the default ones , or the defaults have changed due to version-upgrades etc.
As usual, for the precise details to match your db2 version and platform , refer to the free on-line knowledge centre - all your questions are usually answered there if you take the time...
Regarding your Q3:
you have to specify a compression library when you restore from 32-bit backup to 64-bit system, for example.
The reason is that DB2 uses a compression library from the backup image by default. This 32-bit library is not appropriate for restoring to a 64-bit environment and you have to specify a 64-bit compression library explicitly in such a case.