(1) Facing Qiblah is a condition for the validity of salah. When you know that the bus is heading towards east and the Qiblah is towards west then it shall not be lawful for you to offer salah facing towards east. If the position of sitting is in opposite direction then you should change your sitting position towards Qiblah. If it is difficult for you to offer salah standing then you should offer as you wish and then reoffer the salah when you reach home.
(2) Yes, it is correct to make intention in this way.
(3) See answer number 1 for the same.
Allah knows Best!
Darul Uloom Deoband