Why this "Proc je vsechno tady tak slané" is not correct?
Not sure, it is just a bit awkward.
Buuuut... is there any rule that makes it grammatically bad? I mean, apart from how does it sound.
Well, it's not a rule, but it's not supposed to be like this. Maybe the location adverbial tends to be as close as possible to the verb, but I'm really not sure about that.
What's wrong with "Proc je tu vse tak slane?"
We are missing some synonyms for "všechno".