This to be specific : https://forum.duolingo.com/comment/152063?comment_id=9644863
vorbei = over but it's expected again schluss = ended completely
Vorbei, has a sense of it will happen again, but it is done for now. For example "Ist dieser Monat vorbei?" Vorbei is used here because there is another month after it or like in this situation, there will be another summer next year. Schluss means it is completly done for. So saying "Der Sommer ist Schluss" would be like saying summer is gone for good, there will never be another one.
I don't agree with you. I don't know any German who would make such a difference.
"Der Schluss" is is a noun and therefore the sentence structure has to be changed accordingly. There are some German idioms like "Schluss mit lustig!" (meaning the point is reached where it gets serious -> the fun is over.)
There is no distinction whether something is completely done for or will happen again. You'd say "Mein Leben ist vorbei!" (=My life is over.) although you don't believe in reincarnation the like.
The only difference - I know - is that you tend to say "Es reicht." (=It's enough.) or "Schluss mit etwas." if you want to end something yourself. "Vorbei" is used more generally and neutrally.