1) I'm running the 4.2.1.20 client with a 4.1.3 server, no problems.
I dont' think running with older servers is supported, but may still work.
(Servers older than 4.1 aren't supported, period...)

2) Once the first backup has occurred, no.  Clients are upward, not downward
compatible.

You can restore data backed up by and old client, with a newer client.
But you can't restore data backed up with a newer client, using an old
client.