Why does my Shimano Deore SLX dérailleur not rotate into the expected (vertical) position?
The solution turned out to be very simple - after the first 5° of sweep, if I push extremely hard, to the degree I feel a breakage is inevitable, another ~175° is indeed available. The amount of torque required to burst past this apparent stop is enough to make a 20kg e-bike creak against the maintenance stand clamp! Interestingly if the anti-slap clutch is off, the sweep is even harder to make, and there is almost a scratchy, harsh resistance to the movement. I thought the clutch was meant to make it harder when it is On, not when it is Off, but mine is the other way around.

During my investigations, I tried to remove the fascia plate to examine the clutch. There are three hex bolts, arranged in a triangle formation; I removed two, and managed to wear away the hex key slot for the third, and I may need now to drill that out. I have decided not to investigate this further.

Given my findings, I was finally able to replace chain - I used a bungee cord to keep the dérailleur forward, so the quick link could be fitted. However it feels like it may be worth replacing the dérailleur unit entirely - the chain is now clanking in a way that it did not before, and it is under far too much tension. I wonder if the chain snap caused the dérailleur to fly back so violently that it has broken the clutch.

The dérailleur has RD-M7000 stamped in the metal casing, but a cursory search suggests there isn't much support left for 10 speeds. I will have a hunt about to see what I can find that will fit.