Nobody on these forums recommended DirectShowSource for my script. It was a Youtuber who was told what to do by a contact of his own. Having seen some videos of his, it apparently seems to work alright for him.
In any case, here is a link to a short test video, where the M2TS has been deinterlaced with VD's Yadif filter while doubling the frame rate:
https://www.youtube.com/watch?v=U-hY3XZeRyY
I went ahead and updated my AviSynth to make sure that it was the 64-bit version, and got the equivalent plugins for it. However, I'm still getting VD complaining about that "undefined" on Line 244 of the QTGMC 3.357 ansi file. Don't know how to go any farther than that.