Thou, thee, thy, and thine are just second-person singular pronouns. Thou and its other forms are cognate with tu in Romance languages and du in German. In Early Modern English, you began to be used as a formal form, and it eventually displaced thou.
Verbs ending with -st or -est are the forms that went with thou; when the pronouns disappeared from use, so did those verb endings.
Whilst isn't exactly archaic, though it is pretty rare in America. It's just while with a genitive -s on the end. I'm not quite sure what purpose that serves, but it eventually developed a -t on top of that, possibly through confusion with the the superlative adjective ending -st/-est.
I'm not sure about "many morest." That sounds weird to me. Where did you get that from?