…coffee through a filter is NOT borer. The only thing I remember is that the water goes in and comes out after passing thru the coffee. They were never considered a mixture
It’s a mixture as it sits in the grounds cooking the coffee.
I asked a posek, not someones brother-in-law
Sorry, but I think you’ll need a better source than that. Besides, why does the fact that someone doesn’t have any married sisters qualify him to pasken?
The kli revii is necessary for the bishul aspect
Implied in the requirement for a kli revii is the fact that a kli shlishi would not be okay (which is certainly true according to the Aruch Hashulchan and Mishna Berurah). Since the A.H. and M.B. don’t mention, AFAIK, the heter of a kli revii, it should still be assur (unless the water is below yad soledes bo, in which case even a kli rishon would be okay).