Hi and welcome to the Cosmo crowd, I suppose. Here are my notes on your notes, in the hope that you might find something useful.
The Cosmo doesn't have a row of five RGB LED's anywhere so, as I seem to faintly recall, the idea was that the Cosmo version of Ledison would use the CoDi to show the color patterns, but I have never read about anyone getting that to work and, at this point, I have stopped hoping it will ever get fixed. Since it, on my unit anyway, more than doubles the battery life, and also because mine is so buggy, I have just turned my CoDi off from the settings menu. Before I did so, it kept waking up for everything and anything, such as when AirMail or Google Play were merely checking for mail or updates. Quite often it woke up for no apparent reason at all, and stayed on for 40 seconds.
As far as I know the LED's on the outside of the Cosmo lid works as follows: The quick start guide that came with my Cosmo, says the LED above the camera hump is a 'Wireless Indicator LED'. I have occasionally seen lit in magenta during phone calls, which
I think tells me I only have 2G coverage at the moment. I have seen other people say it's magenta when Mobile Data isn't working, which I suppose it isn't on 2G, so that might be the actual meaning. It turns on in red when the Cosmo is in flight mode and in blue when the mobile hotspot is on (with the lid closed).
The LED in the other end of the lid, below the fingerprint rocker, comes on in red, when charging at below 80% battery level, at which point it turns yellow. Then it turns green at around 95% charge. The same LED blinks slowly with short flashes in yellow when there are unread SMS or MMS messages. I think it flashes in yellow for missed calls too. Mine flashes the same way, but in green, when I have unread Skype messages. It flashes red when battery saver mode has turned on due to low battery level, but not if I manually turn the battery saver mode on.
Then there are the two LED's on the fingerprint rocker itself. Their behavior seems a bit erratic at times. As far as I understand, when the CoDi wakes up in its locked state, both are supposed to be red. After unlocking the CoDi with the fingerprint reader, both of them turn blue. When the CoDi is enabled, but sleeping, they occasionally light up in either red or blue, seemingly for no reason.
While the phone is ringing, the left LED is green and the right is red, to indicate which way to press the rocker to answer or dismiss the call. They are also red/green when the CoDi is showing the contact list, call history or the keypad for manual dialing. In that state, pressing the green side is supposed to call the selected/entered number, while pressing the red side should return to showing the time and date.
When a compatible media player is playing and the lid is closed, the LED's in the rocker are flashing alternatingly in blue, which I personally find so distracting that I avoid playing media on the Cosmo, with the lid closed. This can also happen when Spotify on my Cosmo detects that Spotify on my PC is playing... I got rid of that by turning off "Spotify Connect in background" in the Spotify settings on the Cosmo and then swiping away from the list of recently used apps (the square navigation button).
There is no picture browser on the CoDi as far as I know. The closest thing I have found is that the CoDi media player UI can show album art. I have personally only gotten this to work with Spotify, but I can neither get it to play files from the local file system, nor get it to reliably change tracks or pause from the CoDi. The media player UI detects VLC, but just shows its default cone icon and can't control the playback. Someone mentioned the opensource media player Vinyl, but for me it behaved much like VLC. If some actually supported local media player could be found, creating, potentially silent, local media files with whatever "album art" one would want to display on the CoDi could, perhaps, be a really messy way to show small still images on the CoDi.
Yes, upgrading CoDi firmware is a pain. Flight mode and DND seems to help a bit, but it can still be really tedious. It's just one of the many bugs related to the CoDi, it seems. It would indeed be nice if Planet would opensource the CoDi firmware. The closest things we have, seems to be the projects mentioned here:
https://www.oesf.org/forum/index.php?topic=36060 but, as far as I know, the micro controller they chose is very limited, so it might not be possible to do all that much.
Regarding the keyboard, my Cosmo has, in my opinion a noticeably better keyboard than my Gemini, so my experience differs from yours. On the Gemini the keys were a bit wobbly and unreliable, lest I hit them dead center. On the Cosmo, I initially had a similar issue with the space bar, which would only register if I hit the center third of it, but I fix that by putting a small piece of a thin self adhesive label under the key cap (like so:
https://www.oesf.org/forum/index.php?topic=34914). For a while, my Cosmo typed a double 5. I removed a few key caps in that area, to make sure the rubber mat was clean and flat. Whatever I did, solved the issue, as it was gone after reinstalling the key caps.
The "Battery usage" doesn't work for me either. I haven't bothered trying to solve it, but I might try that GSam Battery Monitor.
I have personally set Settings -> Display -> Advanced -> Font size to small and Settings -> Display -> Advanced -> Display Size (using Shift-Control-C) to Custom (360). In my opinion, that allows me to fit a reasonable amount of stuff onto the screen, while still being able to read even the finer print in most apps.