You don't say in your original post--when I first read this, I presumed that this was on some Rover you just hauled out of the barn after 2 decades of sitting.
Re-reading it, it looks like it may be on a recently-previously-running rover?
If so, then the only way things generally backfire out of the carb is if either the ignition timing or the valve timing is way out of whack.
If it's the VALVE timing, then there's the possibility that something is causing a valve or valves to remain open when they should be closed and a piston or pistons can't rise to their top dead center position because they're hitting an open valve. I'd have a look at the timing chain
I figure that what I described is the worst case scenario and your valves are now all bent up and the piston tops have holes knocked in them from hitting the valves. The good news is that if this is NOT the problem, you'll experience a brief feeling of euphoria.
If the backfiring through the carb issue is an IGNITION timing issue, then it's easy to fix by properly timing the ignition, but it doesn't explain the engine seizure.
Does the engine turn backwards by hand? if so, do the valves move when you are turning it? If so, does the crankshaft rotate then come to a definitive clunking stop? Does it rotate with the starter removed?
Oh, and I hate to ask, but was the transmission in neutral when you tried to rotate the engine by hand?
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).