Ok, I largely fixed most of the issues. Uninstalled AviSynth, direct installed AviSynth+, and then had a new error which was solved by having to paste the AviSynth MT into the syswow64 folder (thanks to this post from Ganfall here: https://www.svp-team.com/forum/viewtopic.php?id=2761).
Now the only issue is that MeGUI, when I go to use the script to encode a video and click to open with DirectShowSource, I get:
which is weird, since I still have the DirectShowSource.dll file in the MeGUI Program Files x86 folder, so idk where it's attempting to look for it.
I CAN encode the video with MeGUI's One-Click method though, so I guess that will have to do for now, unless someone can maybe figure out what the issue is.