Getting back into it

You're selling the car, right?  Take out the system and sell everything as separates.  That's what I'd do, since that's the only way you'll come out a winner with this deal.  Who wants to buy a car with a system only to find out that the sub is using only one voice coil...or that it's bridged at 8 ohms (and has no SQ in the name)?  That just puts the burden on the buyer to get things squared away.

You probably got this already, but anyway:  when an amp is spec'd as "2 ohm stable", the stabilty refers to per channel.  A two channel amp is stable to 2 ohms per channel...just like a one channel (mono) amp is stable to 2 ohms per channel.  A bridged load on a 2 channel amp doesn't "see" anything from the load;  the load is simply split across the two channels.  An 8 ohm load becomes a 4 ohm load onto each of the two channels.  The output would be the same as the rated 4 ohm stereo output.

Your stereo amplifier might be better utilized by someone for running a set of front components, and that sub would like to meet a mono amp.