You will need to take a copy of /var/log/Xorg.0.log when you are able to draw and have no pressure, when you have the computer booted with the tablet plugged in, and when you plug in the tablet after the tablet is plugged in. This will help us see which xorg module is in use at that time.Now, I have to figure a few things. I've added a few options in xorg.conf in order to be able to select menu and draw in gimp (for example). I've tried using mypaint which is supposed to be sensitive to pressure. Usually I'm unable to draw at all (for an unknown reason yet) but when I got it to draw sooner today, I noticed that there was no pressure at all. It is the same in Xournal.
I also have to figure why the stylus won't work at all to select object (or draw) when the tablet is added after the computer is booted but it works when I boot with the tablet already plugged in (then, I'm loosing my external mouse instead...)
When there is no pressure, it could mean that the pressure is being reported differently (so a modification to the source will be needed for your device if we are able to find the new pressure location). It could also be that xorg is using a different module so it does not read the pressure information.
When you lose your external mouse, it sounds like there is an interrupt conflict. Have you tried plugging in your mouse where the tablet is currently being plugged in and having the tablet be plugged in where the mouse is?
The patch that I posted does have an extra print statement in there to try to capture the information like the pressure. If you are able to add that line and check /var/log/messages, we might be able to get some information from it.