I have nothing
Don't see how zero psi will work other than the HP pump is able to suck fuel thru the LP pump, at least your not seeing a vacuum on the low side
Clues I'm seeing
Watching your vid I see you pump the throttle when it dies prior to restarting.
1) Once the boat is running the first time the carb fuel bowl should always be full and not need pumping. Even if it does need a bit more fuel pumping it more than once might cause a flooding...
Opinion: manufactures defects happen and they are not often they happen at the same time or same item. While changing both "might" guarantee this will not happen to the other motor, IMO it's a one off.
Once the drive is removed it's disconnect anything that attaches it to the hull. You have pretty much listed all that.
Many remove the bolts holding the front mounts to stringers, but I just remove the nut on top and leave the mount. In either case have to use alignment bar once motor is...
I hope the bore scoping shows something, but with it being unable to turn by the crowbar in either direction, I'm thinking seized bearing. If a rod broke or crank broke it should still move some if not one direction but would the other even a small amount.
Not as bad as a Silverton but did...
Glad your marina guy didn't break the crank shaft bolt
Don't know how many times someone has said, put a socket on the crank shaft bolt and see if you can move the motor, then they have yet another problem when it snaps off.
Hope they can find and fix the issue at least cost