(
sleep 3
xrandr --newmode "3840x2160_60.00" 712.75 3840 4160 4576 5312 2160 2163 2168 2237 -hsync +vsync
xrandr --addmode HDMI1 "3840x2160_60.00"
xrandr --output HDMI1 --mode "3840x2160_60.00"
)&^ Try that. It runs the commands in the background after waiting 3 secs for X11 to start. Even if 4K60 is working in nightles your issue is the EDID data read from HDMI does not provide that modeline, so you'll still need to add it.
You can put the commands in a simple script and then order the execution using a systemd service file in /storage/.config/system.d/ .. it's been a decade since I used LE on x86_64 hardware with X11 services so I have no idea which services you need in the after criteria, but this is the only way to say "after X11 services start, before kodi.target starts"
NVMe will be faster and more reliable than a USB stick.
No (unless using a test build with HDR patches), No (because DoVi is closed-source garbage), Yes (assuming the hardware supports it).
Same, using Estuary. So it's a skin thing.
AVG is garbage. End of lesson.
Press pause and the OSD appears. There should be a cog icon on the right side, which brings up video settings. Is that what you mean?
I'm not at all familiar with ALSA, outside of being a simple end user, but is that something that could eventually be auto-configed/make it's way into the GUI (assuming a proper arrangement can be figured out/found)?
Kodi has a GUI option to upmix 2.0 to the current speaker configuration, e.g. 5.1, so IMHO the best approach would be a similar Kodi feature that upmixes mono to either stereo (default) or a centre channel in a multi-channel configuration where FC exists. Fiddling with alsa confs is probably do-able but not something I'd ever want to adopt for distro features as it won't be replicable to all devices (alsa confs are device-specific).
Please share the whole dmesg log (via pastebin). It's not clear which LE image you are using.
Kodi has a function to change resolution (which is poorly documented and may not work) but the design assumption is that you either leave the refresh rate at the 'desktop' frequency of 60Hz and let Kodi adapt the content for best output, or let Kodi switch to resolutions that match the media playing (which is better). You can manually set the desktop refresh rate but there's no function to hot-swap/force a specific rate.
It's not the first time someone reports an issue with Denon AVR 
It should be a Kodi feature we inherit and not something specific to LE, hence there's not much ambition among staff.
Please test the current nightly image from https://test.libreelec.tv
I know that there is no Kodi Plugin for this, but maybe this can be solved on OS level? I'm not this far into Linux to develop this from scratch on my own - I'd need to rely on a pre made solution for this. Am I the first one asking for such feature?
The Linux audio subsystem (alsa) is very flexible, but thereinlies the problem. It's so flexible (and technically dense) that you can almost never Google the article with precicely what you're looking to achieve written clearly; it's what Harry Potter would refer to as "one of the Linux dark arts". So you're not the first person to look for an EQ function but there's no Kodi plugin, nothing native in LE, and no mortal human-readable HOWTO guides that I know of ot deal with it at alsa levels. Yay, we 
alsa!
The long-standing (since stereo was a new fancy thing) convention for mono output on a multi-speaker system is to send it to FL/FR so that's not unexpected. The HD formats are not mono, they are always multi-channel. So what you describe as "DTS 1.0" is more correctly described as 6-channel DTS where FL/FR/RL/RR/LFE channels have inaudible content and FC channel has audible content. If you disable pass-thru and have set Kodi to downmix to 2.0; that's what Kodi does, it downmixes multiple channels (including the one with audible content) to the default speaker map for 2.0 audio which is FL/FR. So everything working normally and not a bug.
It's probably possible to use alsa confs to define a multi-channel arrangement that remaps true mono to a multi-channel FC-only arrangement but I don't have any links to share and it's a complete bitch to Google for anything alsa related - all you ever get is a bazillion "not quite what you're looking for" results returned. I'll duck the opportunity to think any further about that idea.
My $0.02: Claim the sofa sweet-spot equidistant from FL and FR and then it all sounds like it's in the centre anyway 
Clean boot an LE11 image and run "dmesg | paste" and share the URL so we can see the full log. From this we can see if there's any evidence of missing firmware or missing driver. The snippets you've shared are not showing much.
Good boot log must have a typo .. it's not a log. Please check when redoing.
Please run "journalctl | paste" after clean boot and waiting ~2 mins after the Kodi home screen comes up. I want to see URLs for two logs; one for a good boot (WiFi working) and one for a bad boot (WiFi not-working) to see if there are any obvious issues. Test with LE11 as that's using newer drivers/kernel/iwd and should be the target for any changes (if needed).
3. Kodi stores all references to sources and file-paths in an sqlite database (or MySQL for shared DB) so you can always move the content to a new drive and then search/update links in the DB to point to new locations. This might require a bit of experimentation (learn to backup the DB files before you start to permit easy stop/copy/restart of files to revert mistakes) but it's probably not that hard and someone somewhere has probably documented it in the Kodi forums already.
