But how can you get a descent lunar lander, capable of landing ~10 tons
on the lunar surface, into the 5.2m dimater faring offered by SpaceX
(or Boeing, LM, or the Stick)?

Can this be done without orbital assembley?

Sure. Why not. 5m diameter is plenty if you do not use hydrogen. Two 4m
diameter spheres filled with liquid methane and LOX would contain more
than enough fuel to land and launch quite a large payload on the moon.
You would just have to fill it up at a propellant depot in low earth
orbit.

One idea I had would be a lander that consists of two propulsion units
that would fit either side of the payload, and would be joined across
the top by a "bridge". The payload would fit in the middle, suspended
from the joining bridge. This bridge would be telescopic, enabling the
two propulsion units to be launched together inside a single faring.

I don't think that would be necessary. I like the skycrane approach
that is now proposed for advanced mars missions:
http://www.space.com/images/h_msl-skcrane_schema_02.jpg. Something
like this should work even better in the lower gravity of the moon.