So what is your stretching process? I shoved some shoe trees with high insteps in them and after about 5 minutes I could see the leather of the vamp starting to check and crack. I pulled the trees out and slathered the shoes with Bick 4. Several heavy applications a day for about 3 days. The leather was still soaking up Bick, and I haven't tried the trees again. In short, how do I stretch without tearing thin leather?
Every shoe is different. Leather in vintage shoes can be quite delicate. A large shoe tree does not have the ability of slowly increasing the pressure. The device I listed would let you do half a turn every 24 hours or so. If I were doing your shoe, I would use stretcher after applying conditioner. Wait 24 hours. Add more conditioner and do half a turn on the stretcher. Repeat 1 more time and then try on. If not enough then I would use stretching liquid from the inside. and repeat the stretching half turns.
I have done this on a few shoes (initially without any liquids and then with half rubbing alcohol/half water solution on the inside if needed) and have not had issues but of course try at your own risk.