These are some very detail oriented questions!
For the metro- yes, if you can use the tube in London the Paris metro should be no big challenge. The lines designated direction by the terminating station just like in London. As for the carnet packs vs. a navigo card, it has been years since I was in Paris and am not positive. I'll be there next weekend though and will try to find out and report back!
As for supermarkets, monoprix and super U are two chains that are usually quite easy to find. If you are staying in an apartment and not a hotel, finding a supermarket won't be all that difficult in the Paris metro area.
I've never rented an apartment in Paris so I'm afraid that's where my experiences stop helping. Best of luck!






