ImageSeq requires DevIL, which apparently is only listed as an optional dependency.
Code:
pacman -S community/devil
Not sure what the issue with FFMS2 is, aside from not having installed it, unless that's just the recursive shared library problem cropping up. If you install DJATOM/avs2yuv, you can pipe an FFMS2-using script to FFmpeg or mpv without the shared library issue causing a deadlock (alternatively, build FFMS2 against a minimal, static build of FFmpeg that you don't install to the system; then there won't be a conflict with the system's shared version of FFmpeg and anything that links to that).