Year of Hell.
Janeway's not above making an alliance if she needs one.
Here's the thing.
Janeway doesn't just speak for Janeway, or even just her ship.
When Janeway makes a deal, she's speaking for the entire Federation.
If Seska had a diplomatic rank beyond "spy" could she bindingly speak for Cardassia?
Technically, Kazon Space had or was in the process of being Annexed by Cardassia, that Seska had at least the military authority to field commission herself to Governor, Dictator or Legit who technically should be able to speak for the Union?
A treaty between the Federation and Cardassia approved by the Maquis.