In the question mentioned above, it is not lawful for you as per the Shariah to show laziness towards salah merely for those groundless doubts. Rather a chair is usually clean, it cannot be considered impure only due to doubt. Hence you should offer salah with jamah and do not pay any attention to these doubts. اليقين لا يزول بالشك A chair shall not get impure if any non-Muslim sits over it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband