I think there must be some kind of technical reason for the absence of slow-mo/frame advance because in all my many years I've never seen it in any player - apart from old school DVRs/Decks and such, despite users' continual requests for same.
Over the last 6 months suffering through PTV I've used VRD a LOT to fix it. I also noticed VRD doesn't like editing AVC streams when it excels at editing MPEG 2 streams and the reason must be those pesky I Frames. Every frame of MPEG 2 is an I Frame while only some frames of an AVC stream are I Frames.
While editing AVC streams with VRD sometimes it gets confused and glitches out - could be directly associated with PTV material (don't know, never had to edit an AVC stream - until PTV came along).
Anywho - if a Player (any) glitches out with AVC streams during the playback of single frames it wouldn't be good to unleash that into the public. There must be a reason. I don't know what it is, but suspect it's right in that area somewhere.
Plex wouldn't discuss with smelly, annoying users (lol) the reason why they can't do something they deem silly. Perhaps Channels can actually reveal the reason it's not (perhaps easily) possible because I do believe Channels is actually working for us Users. As far as Plex is concerned - we're working for them.