I haven't used effects in a while but here's a writeup about all the variables C2 provides:
Tiledbg uses the plugin setting pf_predraw flag which seems to affect the drawing of effects. I haven't figured out how but the relevant part of the runtime is the renderEffectChain function in layout.js. This is probably where the choice between screen space and object space is used.
Is there an easier way of getting absolute screen-space pixel coordinates?
That way should be right based on what I found for that guide.
If void main is called for every pixel, is it unnecessary to put certain calculations in there?
The only way for it to calculate it once would be to calculate it before hand and put it in as an effect parameter as far as i know.
Why isn't alpha reserved when I only adjust the rgb?
It has to do with pre-multiplied alpha.
I'm not sure about the other issues. There's a lot going on in the renderEffectChain function in layout.js, and it probably explains when object space is used and when screen space is used, but in the interst of time it's probably a good question for ashley.