It does sound like it is working as expected.

I've never tried patching Qt before. Since we are talking Ubuntu I guess the best thing would be to get a hold of the Qt deb package for your release of Ubuntu. Then you would patch that. That could be a bit of pain: taking the deb apart, adding a new patch to the Debian patch control, renaming the deb. etc. There are instructions on how to do that on the Ubuntu wiki. It's the kind of thing someone then posts as a PPA if they've gone to that kind of trouble. Patching and compiling Qt might be simpler but I don't know if it will work if you just use the latest tar release or clone the git repository. It likely would. You probably want to check around and see if anyone has posted instructions or a PPA. Maybe post on the bug report asking for help?

You only need to calibrate if the location of the Pen on the tablet doesn't correspond to where it should be on the monitor. If tracing the edges of the tablet also traces the monitor edges your are probably good. If you need to calibrate you could use xinput_calibrator.