(2) Can we offer salah behind a person who might have done a ‘masah’ on his socks instead of washing their feet?
بسم الله الرحمن الرحيم
(1) After washing the feet in wudhu, if they are not placed at any impure place as per your belief then you may offer salah with the wudhu. But if you arrange for jug or bottle and wash your feet sitting on any appropriate and clean place then it is far better.
(2) Yes, you may offer salah behind such person provided the socks on which he does masah are the socks on which masah is lawful as per the Shariah and they meet all the conditions regarding the validity of masah, as validity of masah on khuffain (leather socks) is agreed upon by Ahl al-Sunnah al-Jamah.
Allah knows Best!
Darul Uloom Deoband