It sounds like you guys are making some real progress.
It's great that you're so dedicated to the project.
If you, or anyone, has a 15GP22 that has gone up to air and isn't otherwise rebuildable I think I could arrange to helium leak test it and determine exactly where the leaks are. I wouldn't be anxious to risk a CRT, that could be rebuilt, by having it shipped.
By any chance is the sealing material that you're referring to F113? A water thin epoxy used for bonding fibre optics into connectors? If it is F113 or a similar epoxy, I do use it for forming vacuum tight connections when I have to fabricate my own, but these are on actively pumped devices.
I don't know that I would recommend it for a permanently sealed vacuum because of its outgassing properties. On the other hand, the exposed area must be extremely small since you're repairing a leak, so it might still outlast the normal life of the CRT.
John