No, you don't need to state invitation, everyone is invited. As long as you upload something to the FTP server with the proper readme file with who you are and a contact email, AND it follows all the other basic requirements (Pascal, under 200MB, etc) then it counts as a submission.

Yes you may post any screenshots or video you like. In fact we encourage it. Your submission entry into the competition, though created for it, is yours to do with as you like. We have no say, except that we get to show it off and use it and screenshots and video captures from it to help promote the community and future competitions.