Duras house's "coalition" was receiving help from the romulans for some time - and they had no problem with it.
We're talking supplies to fight an interstellar war
, NOT some covert stuff - you can't hide something on this scale, T'Girl.
Picard knew Sela was in the vicinity because she was ON BOARD the Enterprise.
About treaty vs prime directive - already answered - your counterarguments are repetitive:
Worf was right there near Gowron when Gowron asked for help as per the treaty; Worf did his part in trying co convince Picard. Whatever Gowron was saying, it had Worf's full support/confirmation - meaning:
As per the treaty, the federation could interfere in internal klingon matters (as well as external matters), IF asked by the legitimate government.
Picard recited the overused "non-interference in internal matters of other cultures" AKA the prime directive.
This was the reson they did not interfere in 'klingon internal matters'.
As it turned out, Picard forgot about his draconian 'prime directive' interpretation when he interfered in klingon external matters.