SamyGO

xfabio wrote: So, I'd like to enable virtual USB to read Samba sharing.
I've read the wiki pages, but I didn't find a solution

Are you sure? Use hotel mode hack and you get samba/nfs mounted, Or you can use samygo all extensions after you root TV too.

1. The firmware version is T-MSX5DEUC 2009.0, is it possible to hack it to make the changes I want do?

See above.

2. If yes, which is which is the easiest (and safest) way to hack it?

You have only one way - hack over hotel mode, witch is not very safe...

so I think hacking over Hotel Mode is the only way, right?

Absolutely.

3. ...is it possible to downgrade the firmware?

just after you root your TV,

Have you dowbgrade the firmware?
If not, take it easy, the wiki T-VALDEUC only. To make the same process on your TV, you need to write right ID of your previous firmwares (eg from T-MSX5DEUC 1012).