First of all, you are looking into Jaz/Japs....because Kevin, SY and Kurt played them? First of all, you should know something first:
Kevin used those guitars on Loveless...and even he had a hard time getting that sound when playing gigs.
SY modded the crap out of their guitars.
Kurt modded all his guitars as well.
If you've ever heard MBV live, I don't think Kevin had much trouble getting an awesome live sound, though not the same as in the studio. When in doubt, play hard & deafeningly loud. Central to the classic Sheilds guitar sound is the use/abuse of Jag/Jazz trem--you simply can't play a Strat the way he does a Jag/Jazz...a Bigsby comes close, but you can't really manipulate it while strumming like he does.
SY's guitars are/were mostly modded to hell, but they use all sorts of stuff besides Jags & Jazzs anywho...it's all about the tunings (& playing behind the bridge--said Thurston "Behind the nut is art school--behind the bridge is rock-n-roll")
Methinks humbuckers were a big part of the Cobain sound--besides a vintage Mustang or 2, seems like everything he played & or broke had 'buckers...
Interesting to see Tim Gane mentioned as a guitar influence--his rhythm work is the backbone of the groop--even if he is usually buried in the mix.
I saw one of their 1st US shows & I can still picture him, 'burst vintage Jag in hand, turning his head from side to side, while I basked in a torrent of Farfisa overtones...good times.