1/ you'll get the best sound with a wired headset (the one included is not too bad) the NIT supports a standard 3.5 stereo jack and you can even get a splitter that allows you to connect a headset and a mic (USB headset bring their own soundcard : it could work if you can find and install the drivers : which is not very easy)

3/ I don't think thunderbird is ported to Maemo and I'm not sure it is well adapted to thinger or stylus use ... I know there is at least one other email client available : claws mail but I think modest is doing a great job !