I have heard of this before and was terrified, but I took the lock down advice and I feel a little better. This reminds me of the old phone cloning from the early days of cell phones, but far less complicated so a dumber criminal can pull this off.
In answer to your question about what ROM I am using, I just use the one from the US play store. I would love to get my hands on the next generation build to see how I like the new layout and color scheme but I am not that advanced.
As for splitting your headphone jack, here I do know a bit but...
Ugh, this is a yes and no type of thing. yes you can but no the UI will not be exactly the same there are some minor differences in layout and your phone or tablet may have more features than a head unit like the ability to report things, like speed traps and break downs, in Google maps.
First...