As much as I’d like to see this happen, I do feel that the JBG line is one of our untouchable lines

I understand what you're saying and why you're saying it, but I think Joshua has earned at least some time on the second or even first line.
There should be no such thing as an "untouchable" line or player.
Coaches and GM are always, supposedly, looking to improve their team.
If Sissons wasn't working out on the third line, I would have absolutely no problem with swapping him with Joshua.

Nashville has no interest in Mikheyev and Friedman.

perhaps not, but do they have interest in 2 - 2nds?