(1) It is wajib to offer fardh salah with Jamah. Hence first you should try to offer salah with Jamah whether it is the Jamah of masjid or in case of excuse you can do your own Jamah with one muqtadi or more.
(2) If you are offering salah alone due to compulsion then fardh salah is necessary with qiyam, ruku etc and fardh shall not be valid without qiyam. Hence it is not lawful to offer fardh salah while sitting. (Yes, if you are unable to stand then the case is different).
(3) However, it is allowable to offer nafl salah while sitting but you shall get half of the reward. Hence you should find any way to offer sunnah muakkadah while standing as well.
(4) Yes, it is allowable to offer nafl salah in car while sitting, but you should see how would you perform ruku and sajdah. If you are offering wearing shoes then it should be clean or you should offer by putting it off. However, facing Qiblah is forgiven in it i.e. after starting salah if the direction of car or the person offering salah turns to some other direction then it is forgiven in nafl salah and salah shall be valid. But in sunnah muakkadah facing Qiblah is necessary and the Salah shall not be valid without it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband