I have similar problems with my 11 Journey.
I agree with the cruise controls, I've only used them for about 5 times, just to test it out. As you said, the buttons don't react quickly. I personally don't like using cruise control at any time because I don't feel connected with the road. And I always think that the car computer cannot decide for the road's conditions.
I don't have any problems with the A/C.
Automatic headlights do take a few seconds to change. The sensor doesn't constantly measure the ambient lighting, otherwise it would be using too much power/energy? Like the journey, it also takes a Mercedes and a BMW a few seconds to update the headings as well.
As for the radio, it doesn't even show me the radio station name in the 4.3" screen.
For MP3's, my system sometimes stops for a fraction of a second between songs, I don't think that's a problem as long as it's not pausing in the middle of the song.
For the Unknown artists display, that's weird. I wonder if my car does that too.
I have the same problem with some songs displaying long duration times. About a quarter of the songs on my thumbdrive does this. But this happens with my parents' Mercedes and BMW too. I think there's something to do with the MP3 file, but I haven't looked into fixing the issue.
I don't have any problems with the search by ____, all songs/artists/albums show up under the search list.
I don't use the speed display on the EVIC, so I don't know if it does the same thing with my Journey.
I would keep the dimmer dial at one level before max (Max = when all lights are at 100% brightness and interior LEDs are on), then even when the headlights turn on, the UConnect screen would stay at max brightness. I don't think the display brightness will ever depend on the sensor on the mirror because everyone's eyes have a different sensitivity to light. And like you said, if it relies on the sensor, if the system thinks that there isn't enough light outside, the system would turn the headlights on and dim all the display to keep your eyes comfortable. Like you said again, the system turns on the headlights when theres still plenty of ambient light; in this case, if the uconnect display dims, you wouldn't be able to see it. (I hope that made sense to you)
I'm just sharing my thoughts (:
Apology for the poor english.