Hmmm, I now see that I flashed the wrong / old Dirty Unicorns ROM by Daniel_HK: in this other thread and post he refers to having fixed the Video problems:
He says in the changelog: Find a way to fix the hwcomposer for video/audio playback, need to review later
And refers to having fixed it in this post:
So I guess (I do not have access to my N7 right now) I flashed his 20160917 ROM whereas I should try this one: aosp_grouper-ota-eng.20160923.zip
So I reckon that insight in howto fix video can be found there.
BTW, flashing that ROM was a fight to get Gapps to work, but in the end that ran and I could test the video apps I pointed out before in this thread. All work: kodi / nos / horizon go.
So now looking forward to nAOSP to include this video fix