>>7981861. i think it depends on the codec, but with VP8 it definitely does not matter. you only need the log, so might as well pipe the output to NUL. afaik ffmpeg wouldn't even write anything anyway, just create an empty container. output format does not matter either - -f webm and -f rawvideo take the same time and produce the same log.
2. interesting question, i've never even considered the logs might be generic. i've done some testing. changing *only* the bitrate reliably produces identical logs (same checksum). applying a scale filter, produces logs of exactly the same size, but different checksums. so the amount of information is not different, but the contents are at least to some degree. changing fps also changes the file size of the log, which seems to be a function of number of frames.
therefore i conclude: varying bitrate off of the same log is 100% safe. scaling video is pretty dodgy, but might be safe-ish. changing fps or doing anything more advanced is a no-go.
3. that could be for a lot of reasons. for starters, post your full command. this really should not happen, 2passing should reliably produce video with the target bitrate, but a lot of settings might interfere. both -crf and -qmax switch libvpx into constant quality mode which restricts 2pass' ability to juggle bitrates. try it with just barebones settings, maybe only specify -b:v (and be aware of the notation issues - there's a difference between -b:v 1M and -b:v 1Mi)
