بسم الله الرحمن الرحيم
(Fatwa: 1958/1418/H=01/1434)
(1) Your practice regarding washing the feet is alright.
(2) If they are khuffain (leather socks) then masah is valid over them provided all the conditions are found. If they are nylon socks then masah is not correct over them and when masah is not valid then salah shall also not be valid. And when the salah of imam is not valid then the salah of muqtadi shall also be not valid. Thus in such case you should not offer salah behind imam in such case.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband