>(1) Would it be possible to restore a database backup from a TSM server
>    on a S/390 mainframe to a TSM server on an AIX system?

Thomas succinctly summarized the prohibition.  I would only add:
The Admin Guide manual, Chapter 16. Managing Server Operations,
Moving the Tivoli Storage Manager Server  is your reference on this issue.

The same considerations apply in this issue as in moving any other kind of data
files across systems and platforms:
 - Character set encodings may differ:  ASCII vs. EBCDIC; single-byte
   vs. double-byte.
 - Binary byte order may differ:  "big-endian" vs. "little-endian", as in the
   classic Intel architecture conventions versus the rest of the world.
 - Binary unit lengths may differ:  as in 32-bit word lengths vs 64-bit.

If you get into client-server programming you quickly come to realize the
implications.  Remote Procedure Call disciplines were perhaps the first to fully
address the disparities, in the eXternal Data Representation suite, to have data
end up usable upon arrival.  In doing network programming, the issue comes up
again, as Network Byte Order has to be observed.

In all of this you can appreciate that the *SM server has to accommodate all
comers, as it stores client data and attributes in native form.  And you can
thus appreciate why you should not expect to be able to display client data and
file attributes from within the server any more than you should expect to be
able to be able to restore the *SM database arbitrarily across platforms.

  Richard Sims, BU