yes everything inside myth is unaffected. When you touch the watch TV button, we simply spawn the myth front-end, and display a remote control that lists the myth commands, and simply sends those commands to myth. We may no changes to myth itself, so all the functionality is the same.
and yes you should be able to use any DVB card. We are also working on incorporating vdr project, which is specifically designed for DVB cards that are able to provide EPG data within the stream, so you do not need XML TV or another grabber. That module is probably about one month away.