MPC doesn't use ffdshow

After installing the latest K-Lite Mega codec pack (1375), and you select ffdshow as the video decoder, MPC still uses LAV. GraphFilterNext will show that ffmpeg is in the filter chain and is being used, but MPC won't use ffmpeg unless you explicitly specify.

I assume you mean ffdshow. One reason could be that you enabled hardware acceleration. Disable the internal decoders (transform filters) in:
MPC-HC options > Internal Filters

Is there a specific reason why you want to use ffdshow? Which of its filters do you need?

Yeah, my mistake. I meant ffdshow. I have hardware acceleration enabled (via installer), but that configures MPC to use the external LAV filters, not the ones built in. The internal LAV filters aren't used, and are also not configured for hardware accelleration (checkbox is unchecked).

I'm using ffdshow to run Avisynth scripts in line and to be able to edit scripts live.

Interestingly, if I add the ffdshow raw video decoder as an external decoder, and set it to "prefer", MPC uses the internal LAV decoder (which has hardware accel off) instead of the external one.

It continues to use the external one here with ffdshow raw video processor.
Internal and external are the same filter. Only difference is that their settings are stored separately (to allow custom settings in MPC-HC).

LAV as decoder plus ffdshow raw video is the recommended combination if you need Avisynth.