Inspect face-frame inside pocket screws on inside face of face frame, and consider removing frame, or part of same, to gain access? Any Tech will have to do similar, sounds like?
If removing a few pocket screws do not give a release, then oscillating tool maybe very handy? On mine, the two bowl w/ small/ large bowls, my only access was drawer removal, with electric panel and flex duct and wiring under sink, with stem leak.
I realized I needed to fully install upper bonnet to fully seat the cartridge, then snug up bottom nut to countertop, or bonnet nut could not fully seat the cartridge if bottom tightened first, and thus would leak. BUT, I was able to reach bottom nut with drawers removed. (ON PLASTIC FAUCETS= No tools required except screwdriver for knobs.) I suspect your upper bonnet was loose, and/or your regulator failed or was not installed, which allowed entire cartridge to blow out?
2) Removing drain and shelf may also be helpful or required? Take PICS before disassembly. 3) Other options to consider/ explore might be sink removal, or full countertop removal.
Good Luck, probably/ hopefully easier than you expect.
2000 Conquest LE 6266, Class-C on 99-E450SD, 6.8, 2v, V-10