[dspillett]

WRT b: surely using fuel that is above the engine will bring the centre of mass down, because the engine's mass is still there with less fuel mass above it?

Unless the rest of the vehicle's mass (all the other equipment, and crew if it is a manned mission) has more mass than the engine & landing apparatus of course, which I think (caveat: no deep thinking involved here) is likely for manned missions but less so for others? I'm assuming the mission mass is above the fuel (having the fuel on top would presumably be less safe/reliable/practical/other).

[jvmboi]

I could easily be wrong and I am very open to learning as what I wrote is just my intuition developed from playing KSP.

The Apollo's "lunar module descent engine" weighs a mere 180 KG vs the approx. 4200 KG of the rest of the craft (dry mass). Just the fuel for the descent is then roughly 8000 KG.

Obviously, landing on the moon is possible but I do think that the inherent requirement to have engine(s) and fuel tanks below the payload makes landing in a vacuum a bit of a challenge.