1. Full timers get 4 weeks leave plus 2 " duty free weeks", so without going into the arcane details call it 6 weeks.
2. BA can't "swop" you onto anything....in theory at least your final roster ( published just before the start of the month) is exactly that...that said in the event of disruption once down route all bets are off (which I guess is the same everywhere).
3. Can't answer that without knowing what you've heard.
4. You may get the leave you want, you may not, you bid for it but what you get depends on the demand for those particular days and also how much high value leave you've hoovered up in previous years. Outside of leave you fundamentally have zero control over your days off regardless of how long you've been in the company. That said you may be able to swop to get the days off, than again maybe not.
5. Assume EASA limits with not too many blocks of days off at base.
6. 6 months.