One who cannot perform salah standing and can perform ruku and sajdah, it is better for him to perform salah sitting. As the salah is performed sitting on ground with ruku and sajdah, in the same way it will be performed on chair as well.
(2) In this case, performing salah sitting on ground is better.
(3) Yes, the salah must be performed again.
(4) If someone is unable to stand up, he can perform salah on chair.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband