Thanks, Squirtdad, rosefarts and XxHamBondxX, for the additional input/ideas.
I've found a bike repair shop I'm going to bring the bike in for an evaluation tonight, and am sharing with him some of the most specific suggestions from here, that I tend to agree with.
And to Squirtdad's question about the last photo, showing a white undamaged frame at the top of a seat tube: yes, this is the exact same model bike, undamaged, for general reference.
I was sad to hear your concern that the crack is more like a "fracture", and examining it again with that observation in mind, yes, that may be so. That suggests to me (not being an expert but also not being wholly ignorant of physics) that the quality of the AL used may have just been slightly inferior, or that I just really wrecked that section of metal, and it is indeed compromised.
However, I do hold out hope that it can be repaired to a perfectly safe state, with the right combination of seat mast length insertion, maybe some minor welding, and certainly some ugly-but-effective clamping efforts. The repair guy I've found is very experienced and has many excellent reviews from customers, so we'll see. If I get this thing up and running again and feel good about the safety concern, I'll try to post "after" pics here, as another thanks for the help received!