I appreciate the additional suggestions about finding how-to instructions in print or video.
I did decide to temporarily deal with this issue with Bars Leaks, and I’ve been reporting on that here: Bars Leaks for radiators - how well does it work & perhaps last?
I have not had time to investigate replacement further but I hope the patch effort lasts long enough for me to get around to replacing this tube assembly.
Thanks for reading & responding. I will jump back in after replacement with a post-mortem on the effort.