Re: there's been= there have been plural
I would put it in the category of things we say, but things we shouldn't write.
Just like we say "There's... " before something clearly plural, but would usually write "There are..." I''m sure people often do say "There's been.." before something that's clearly plural, but would usually write "There have been."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.