Hi goof !
I can’t answer all your questions but maybe some.
Ubuntu / ubuntu mate are made with some toolchain which generate ubuntu packages for armhf/armv7 architecture.
This is a generic compilation for that kind of CPUs so the packages are not specifically done for rapsberry pi.
Some kind of software like vlc or ffmpeg need some additional compilations options to activate hardware acceleration decoding which are not provided by the toolchain.
I think ubuntu mate for raspberry pi makers must recompile this kind of packages with the good options and provide them .
=> I don’t know why they don’t do it already !
I compile myself the last version of ffmpeg with hardware acceleration decoding.
So I can read HD videos with ffplay, it work good but I have audio delay between video and audio.
I have the same issue on all video players : vlc, mplayer, mpv and the sound under ubuntu mate is bad on my pi (choppy, sometimes halt …)
Have you audio delay too when you play some video with vlc or with chrome on youtube with some 360p video ?
I will try to recompile vlc with hardware acceleration decoding if I resolve my sound delay issues.
For playing some html5video from youtube you can see my tutorial it’s not the best but it work :
=> Ubuntu Mate 15.10 : Browse Youtube fast and read videos with VLC
For flash it’s complicated, flash support under linux was abandoned by adobe few years ago.
The only support today is provided by Google Chrome which provides flash plugin embedded.
But there is no Google Chrome for arm architecture only Chromium which don’t provide flash.
But there is a way, there is some chromebooks under Chrome Os with arm architecture which have Google Chrome. So it is possible to dump the flash plugin from them and install it on chromium in ubuntu mate for rpi.
I did it, it work well, I will write some tutorial about it until the end of the week I hope and post it here !
But the flash plugin don’t have hardware decoding acceleration too, it’s only sofware rendering but work good !
If I can fix my audio async problem reading videos, maybe I can provide some good solutions here !