Can anybody buy this character as one of the Federation’s best negotiators?
Sure - as the point is that he's a has-been. And of course one who doesn't realize he's a has-been...
Which is why the body rejuvenation schtick is all the more effective here, as it highlights how futile the quest for youth really is - you can't undo decades of mental decline just by re-introducing physiologically sharp wits.
"A Private Little War" had lots of location shooting; "Too Short a Season" spends much of the time being a bottle show, although admittedly the underground corridors are convincing and cool. The view beyond Karnas' window is a weird piece of wasted effort, as the intricate physical model and matte work apparently doesn't look much better in HD, either.
I liked the weaponry seen here: old phasers mixed with what could convincingly be Klingon export-rated guns, made of their rifle butts but with humbler "end effectors".
On the plot side, the general storyline works for me, and the final confrontation is okay, but it's all very predictably theatrical from the very start: "two bitter old men pitted against each other" is going to end in a coughing and wheezing twin monologue in a single (wheelchair accessible) room with dramatically dim lighting...