Q1 - Firstly, forget about the teenager (they won't want to sit with their parents anyway and will probably be quite happy on their own wherever they are). One of the adults can split off if there is no other choice - what you're really looking for is 2 seats adjacent (Parent with inf on lap and the kid). Solution is to look around for likely looking pax and then ask them for help: Use bribery with free drinks, smiles, anything that works - there's usually some kind soul that will help and some miserable child unfriendly B******s that won't. Failing that, try a PA, pointing out that the aircraft can't leave until all pax are seated (careful here, you're trying get pax to respond appealing to their better nature rather than browbeating them into feeling guilty). Some aircraft are more critical on weight/balance than others: Ours aren't, and the pilots are busy pre-departure, so I wouldn't trouble them with a trim change for these few people. Anyway, if the flight is full, there is no change in weigh/balance, and if it isn't, then the reseating will be easy anyway.
Q2 - Decide which one of the two you fancy the most and then thump the other one!
Easy when you know how
