1)yes, if you're not needing 10bit, then AVISynth is fun and does some amazing things. Even it's features of doing technical restoration were useful, but once you step into professional, the 8bit only really holds things back and it was time to move on. FYI, a lot of the AVISynth filters are now available in FFMPEG!
2)gstreamer compared to ffmpeg to me personally is like comparing GIMP to Photoshop (dropping commercial vs FOSS). Yes, GIMP can do nice things, but Photoshop does what I need and I know how to make it do it. No questions asked. Muscle memory. I'm at that point with FFMPEG.
3)I'm long past trying to do creative things CLI that those 3rd party plugins allowed. I'm utilitarian with workflows and need to churn it out, so ffmpeg plugs in the holes as if it were designed for it ;-) If I'm going creative, I'm in a nice UI and doing things. However, I'm not afraid to use CLI to generate some content to messing with later in a more interactive environment.
2. The closest equivalent to VFW in linux is not ffmpeg but gstreamer. My experience with the latter is ... not good.
3. ffmpeg is great when it does what you need but VFW lets you draw from a vast ocean of third party plugins that do all sorts of niche things.