Hmm that's weird. I am just converting James Baker's Arcade video to HEVC, gonna be another 4 minutes, i will post size comparison here when it is done.
You can set all your settings then save it as a preset that auto loads when you open the program, then you browse to a file add to queue, browse to the next file add to queue etc.
Yeah they will work fine, as long as you are compressing to a usual format like mp4 or mkv, Handbrake will even convert to hevc if your pc can play those files, and they are even smaller files with no loss of quality.
Just load a game and hit tab on your keyboard, then choose "input (general)" then if you scroll a way's down in there you will find all the "P1" controls.
What number romset are you using? You say you are using mame version 0.190 are your roms the same? Also which games have you tried that won't load, don't need a whole list just a few.
I am not sure if that is actually a released theme or not. It doesn't look familiar to me at least as one of his released themes. He is always tinkering with themes so it might just be a quick mockup to demonstrate the custom controls he made. Only @Grila would be able to answer that for you.
Just go to your arcade platform on the left of launchbox, then click on a game then ctrl a to highlight them all, then go to tools/change roms folder path.
Yes that is correct there is, but MAME also needs to know where your roms are, so if you have just moved them you will have to tell MAME the new location also.
I would also check first that your games launch OK through just MAME by itself, if they do then we can narrow it down to a launchbox configuration issue. If they don't load through just MAME as well then we would need to troubleshoot there first before even starting on your Launchbox configuration.