How much mass had to be lost before the LM could get to any orbit with half fuel? 1000 kg?

About half its mass. Δv is a function of the ratio of the fully fuelled mass to the empty mass. If you cut the fuel mass in half, then you have to cut the empty mass in half in order the maintain the same ratio.

I just realized something important when I re-read this thread. The RATIO of descent fuel to the mass of the complete LM and the ratio of ascent fuel to the mass of the ascent stage is . . . . .

the same (close, anyway).

Yep, that's exactly right. Good observation. Since both descent and ascent systems had to provide about the same delta-v*, and since both system has about the same specific impulse, both systems required about the same mass ratio.

* The descent stage actually needed a little more because it spent time essentially hovering as it came to a soft landing. The ascent stage could just takeoff and go and not fight gravity as much.