Since your original message I noticed an update for the 7750 in the 6.41RC changelog:
ppp - added support for Sierra MC7750, Verizon USB730L;
PPP not LTE, unfortunately, and I'm not exactly sure what USB730L references. But maybe it means you can get your 7750 working with PPP at least.
Also, if you didn't already, try the program I listed earlier for the DIP/QMI switcher. I assume it wouldn't work at all if in QMI mode, but you never know.