How can Sela win? Simple - by doing what she was already doing
- by continuing to help help the Duras house against Gowron. No need to face the entire klingon fleet.
And when, with romulan help, the Duras house will win the klingon empire, the new klingon-romulan alliance could go against the federation.
No element of this plan is impeded by the fact that Picard can see her on sensors, as opposed to only knowing she's there.
T'Girl, once again, Gowron and Worf cited the treaty
when they asked the federation to help the legitimate klingon government against outlaws - these guys know what the treaty holds (unlike you), meaning the treaty allows for help against such outlaws (much as it, apparently, allows for help against external threats).
Picard refused due to non-interference in matters of another culture - he recited the prime directive - giving it a draconian interpretation.
Later on Picard helped Gowron against 'external foes' as per the treaty. The non-interference of the prime directive would forbid this, too - interfering in romulan matters? are they excempt from the prime directive?
Well, Picard broke his prime directive interpretation with no hesitation.