All are welcome (to the party). They are all invited or free to attend. All are welcomed (to the party). This sounds a bit unusual but would be the passive with the past participle of "to welcome" with the meaning that each person has been welcomed by someone.
Yes, you're very close Nugso. "You" is required, but "all" is optional. Here are the ways I might respond to several "Thank you"s:
You're welcome! You all are welcome! You're all welcome!
The contraction is preferred. I can't promise the last example is grammatically perfect, with "all" after the verb, but it sounds best for everyday use.