I’m not sure if this will solve your problem, but I managed to
solder a difficult delicate chain today, by soldering a tiny sphere
onto the end of the chain, wrapping a thin collar of wire underneath
the sphere and soldering a jump ring onto the top of the sphere.
This seemed to solve all the problems - I should add that I used a
laser welder to tack the components together - this made it a lot
quicker, but it should still be possible to do it without one.
The idea is that the jump ring will solder onto the sphere better
than it will solder onto the chain, and the sphere + collar is easy
to solder securely onto the chain. If I’m making no sense
whatsoever, I should be able to send you a photo of it next week,
when I go back into the workshop.
I even managed to do it without fusing the chain together!