I went into plyrgeo.viv and renamed uneck* to uneckwide* and vice versa, then switched all the 5's and 0's in necktype in teamgear.dbf.
The normal, classic, "uneck" necktype for Boston, NJ, Minnesota, now works correctly even they are playing against a team with a differnet neck type.
Interestingly even though I switched the Boston alternate to '0' which should now correspond to the uneckwide, it still has the normal uneck shape - I think the game reads necktype=0, takes it as a null value, and refers to a default model from somewhere, which happens to be the normal uneck; this probably is where the bug comes from in the first place, if the 2nd team has necktype 1-5, that one replaces the default one, and gets pasted onto the team with necktype '0'.
Luckily for this workaround, the only jersey that uses the uneckwide is the Boston alternate, and necktype '3', vneckwide, is close enough that it looks fine if you just change the necktype to 3; essentially this workaround removes necktype 5 the uneckwide from the game, but since it's only one jersey and model 3 works well enough for it, it's no big loss.
I'm going to upload this fix as well as an updated teamgear.dbf once I tweak the Celtics auerbach patch jersey patch I made, and upload them all together, probably tomorrow.