Wow, lots of discussion to see when I wake up, thx :D
You've all got some good comments, and @rek I do agree with you about the double blockiness happening with a-c. The main problem with (d) is its a lot more expensive to do, so if I could find a non-blurred solution then that'd be better. I guess I'm fine with it being blocky, as long as I can blend the shading with the tiles better, so as not to reinforce their blockiness more than necessary.
The main problem I see with (a) and (c) is that you get those annoying banding artifacts from pixel art when you have gradients of pixels/blocks lined up next to each other.. In (c) I tried to reduce it by blurring the lighting together a bit and then re-blocking it, but the diagonals are especially annoying to me.
As long as I can achieve some semblance of Mood and Discovery with the lighting then I'll be happy. I might go with a variant of (b) for now until a better idea comes along.