I'm afraid that it is, indeed negotiable, and this long-time jazz educator who started out as a jazz player prior to becoming edjumacated and sheepskinned would like to negotiate it..
That triplet equivalency you detailed is really only accurate at certain tempi. If it was accurate across the board, programs like Sibelius, Finale, et al, wouldn't have the ability to choose the "swing ratio" or "swing proportion" of just how much time to spend on the first and second written eighth notes of each beat.. there'd be no need for it.
A simple exercise to prove my point, for those with access to a sequencing program, Sibelius, Finale, MuseScore, or anything that can change the speed of a recording you already have. Program in a figure in 12/8 time that is a repeated quarter note/eighth note figure (the equivalent of what you describe... a triplet with the first two notes tied together). Use, say, the melody from Donna Lee, or Ornithology, or any other notey bop-head with a lot of running eighth note patters. Play it at a metronome marking of dotted quarter = 110, say (or quarter note, if you've programmed triplets in 4/4 instead of 12/8 time). It'll sound accurate, but still a little weird compared to a good player's version of swing.. the two-to-one ratio rarely sounds EXACTLY right... but OK, close enough here. NOW... set the speed to twice that. It'll sound completely stupid. Nobody plays like that. Vinyl afficianados? Go ahead, play a 33 1/3 record of a medium tempo swing tune at 78. Ignoring the chipmunk affect, the rhythms will sound completely stupid.
"Swing is a triplet feel," "Emphasize 2 and 4," and "Lay back" are all phrases non-jazz players are initially taught, either as legit players dabbling in jazz while pursuing a music education degree, or as young students in public school. While there is a grain of truth to all three statements, they are tools to an end, rather than words to live by. They are vast oversimplifications to introduce concepts that are difficult to comprehend for non-jazzers, that's all. Jazz is something you have to hear to learn. No other way to do it.