Massallah regarding making a mistake in Salaah

as-salaam-wu-alaikum, mufti-saheb

if one makes a mistake in umri-qaza salaat, do ypu make sijdah-sawh or repeat salaah?
if you are in the state of wudhu for some time (eg. one-hour), and you put on khufs(mojas) later; then is that wudhu still valid?
also if you had wudhu with khufs on ,and then take out the khufs, will the wudhu still be valid?
jazakallah