There was also a rumour about JAL moving into T5, but does T5 have the capacity for it considering T5 still uses bussing gates ?
Nowhere near the capacity. To get other airlines into T5 you would have to kick out BA's own services across to inferior facilities and the bussing gates at T3. Also there is currently no provision for IB and JL's handling agents (Menzies and JAL in house), unless BA Heathrow were getting back into the handling business a decade or more after leaving.