1. Just ask! completely appropriate to say "Hey this is K0NAK, didn't quite catch the call can you repeat that please?"
2. Some nets almost sound broadcast, but it's just dudes on the radio.. you could still break in technically. I've heard that one too, with the for sale section towards the end?
3. Usually means they're hopping off.. if sticking around but just done transmitting I'd usually say "monitoring," or to really geek out I'd say "K0NAK clear, but continuing to monitor."
and here's some CB etiquette for you to brush up on..
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50