Looks like a great project! As for how you get mitres like that with paper templates, I do them all the time for seat-stays 
My template program (which I wrote myself) has an “offset” option. It can also do tapered head-tubes.
The code is linked here if anyone is interested:
The only mitre that I’ve really found basically impossible to do with paper templates is the top of unicrown fork legs. They’re all curved so you can’t wrap the paper around, and it’s often at an interesting angle. For those I made a fixture that can do them with a hole-saw (I don’t have a mill).
