NOW to your questions.
How can she be a cougar if NuKirk's smitten-ness is unrequited?
He wants her to be a cougar. He wants her to be HIS cougar. He wants to be her romantic and sexual prey. He looks at this beautiful, powerful, very very self assured woman who is older than him and that is what he sees--a cougar he never knew was his greatest desire until he laid eyes on her. You know sometimes you don't know what you want until it appears before you.. the cougar word is just convenient because it conveys power and the age difference. It's not like this is a word Janeway herself would
ever use.
Do you want her mercilessly flirting with the young Captain, or do you want her on a pedestal, head and shoulders above the young man, flirting with everyone ELSE as she ignores his obvious "attraction"?
I don't want her flirting with anyone, she has a job to do here, many many lives depend on her. She is quite used to compartmentalizing the woman who has desires and needs from the woman who has great responsibilities. But you know, despite NuKirk's tongue hanging out all over the place and his open interest in her which she fobs off over the course of this mission they (presumably) engage in together she comes to deeply admire him. Admire as a real person, not just as the great Captain Kirk who is a piece of history.
And then, with a shock, she realizes there is more there than just admiration.
On the one hand she sees that element of Marla McGivers at play, because many of us DO admire the great ones of history, idolize them and feel the attraction. But being Janeway she is able to turn that over in her mind, name it for what it is and put it away as a curiosity. What surprises her is when she realizes her attraction to NuKirk is far more personal than hero worship and a hot young man, she finds him deeply personally attractive. And that's a very sad thing as she realizes that in another life, another time, this would have have been her greatest love, the man she has always wanted. And she can't have him because she is separated by far more than a couple decades.. and before she returns home she passionately kisses that man, that should have been true love, goodbye.
(OMG this is so sad)