Andrew,

but does this memory usage increase with each loaded clip/media, or stay the same? does memory actually leak (usage grow, and some memory still not freed after program quit)? 

18% sounds a lot (too much for single-frame decoding i was hoping for) 

Yes, it gradually increases with each loaded clip/media.  The memory is freed after the program quits.
I even tried this line in ffmpeg.C instead as a really wild guess:  av_frame_free(&probe_frame);
For now, leaving the ff->video_probe(1); line commented out.  Someday we can get back to it.
Do not worry about it -- something will pop up later.  ...Phyllis