> 1)  Is there any way sender who was served off a list can test
> whether he has been effectively served back on without sending a test
> message without everyone else on the  list seeing it  (The THANKS
> command does not seem to apply here.

Serving off applies to the whole LISTSERV server, not any particular list.
So if you get a response to a THANKS command, you are not served off.
You still could be filtered, however, for a particular list.  Looking at
the list header with REVIEW might give you a hint, unless the filters
are hidden.