I haven't followed this whole thread but it appears as no one mentioned the tried and true way of stretching. If I someone mentioned it and this is repition, sorry.
The way to do it is to take a new tire and mount it on either a new (unlaced) or beat up(and cleaned up) rim. The rim is to have no adhesive. Manhandle the lightly inflated (enough pressure to make it round) tire on the rim. Pump it up to 95 to 130 psi (depending on the pressure limit recommendations) and let it set for 2-3 days.
If it doesn't blow during that period, remove, deflate to 40-60 psi and place in a dark cool place until just before ready to use and then remount 24 hours before you start working with the glues.
Nobody told me about this when I was new to sew-ups 35 years ago (at age 14). It took me about 1.5 years to figure it out. In the interim I tried mounting glue prepped virgin tires on glued prepped rims and attempted to wrestled them on 'till I was sweating profusely, cussing under my breath and/or ready to cry. Lacking solvents to clean up the mess I created I rode my bike with messy tires and suffered through glue covered hands and destroyed shirts until I figured it out.
Todd Teachout
Hercules, CA