That is normal and expected behavior. Up and down keys are default to seek 30 secs for mpv, left right is 10 secs.
As i stated, the app is meant for touch screen not physical button navigation. It is gonna have issues on unsupported devices.
I have ausb RF remote on my shield that plugs to the usb, it is a air mouse remote, u can also use a mouse to get a cursor on Android or ATV (bluetooth mouse can work too if u have no usb port). that i find is the only way to work the OSD on ATV with no touchscreen.
there is a toggle icon on the OSD bar to switch from HW to SW decode. if u have HW enabled in settings, and is is using SW, then it failed to load HW decode, Pressing the OSD button triggers a force use HW, but it may just do nothing, or it may trigger a app crash if the hardware decoder is not compatible.
U can also press "i" as in Info, if u have a keyboard attached ( my media remote has a mini qwerty keyboard on it) and it will show an OSD live stats of the video, bit rate, fps, render dropped frame etc. (this live OSD overlay ability or a "stats for nerds" i have requested Channel have, Emby and Plex have it)
Again, i have never tried it on a TV, only on my Pixel 3 and Shield and PC.
And i only use it for playing local .mp4 or .mkv files.
(the Debanding works fine on my Pixel 3 btw.)
I very much understand your reasoning and frustrations with that statement, that mirrors what experience I had with Emby when i wanted to use it on my Shield for Live TV. Same drawn out thing, and in the end, ws told, nothing we can do really...so no point asking anymore. Shame, i actually like Embys more mature and Advanced UI better. ( i still use it to play my collection of media as a home media sever from time to time)
The dev already stated, it is an issue with the Sony TV AND how Channels software interacts with that specific hardware.
If it was a Channels issue entirely and solely, then others using other hardware would have the same specific issues as you.
{A similar situation with specific device compatibility or quirks arises, for example, with YTTV and using a Nvidia Shield as the Channels DVR server. If you use that setup, then u can NOT use YTTV as a TVE source. That is due to a limitation/issue with the Shield device it self that Channels can not control or fix cause it is locked code in the Nvidia ATV ROM on the device. (something to due with the bundled Chrome build Nvidia uses in it).}
This is good to hear...maybe some progress can be made then if software decode is working for you.