Thanks @mautz for the help. Just some thoughts and feedback (in case others stumble across the same problem).

1) When installing mce-tools, ignoring the dependencies is not a good option. It'll give you a phone the freezes constantly. However, it's possible to intentionally install an older version of mce-tools from the same repo which is compatible with mce on the phone.

2) As for the bluetooth car sync problem, I am afraid I couldn't find anything.

3) Out of curiosity, have those who have version-dup-upgraded to 2.1.4.14/.15 experienced any problems with their Nexus 5? Just to confirm, I am really using this phone (and only this one), thus I need it to be reliable.

4) People calling me keep complaining about an echo (on my end, everything sounds great). I remember there was advice given for the CM12 based SFOS. However, that didn't fix my problem back then. Has anyone tackled that?

5) Additionally, that bluetooth handsfree problem (illegible audio when connected to handsfree device) has been bugging me right from the beginning. After all, if that worked, the phone would be almost perfect. Googlin' hasn't got me anywhere so far (mostly for lack of background knowledge). I hear when the Nexus 5 was first released by Google, it had the very same problem under Android which they fixed with the first update. Isn't there any way to find out what exactly they did?