Okay, I think I've got this, let me just make sure!
Kubfu learns a different fighting style instead of the water/dark ones based around sex, when she's learned enough SEX FU from the MC, she evolves into Urshifu.
I might tweak the end part a little where the fight is against two traditionally trained Urshifu owned by the Dojo, Kubfu can use her new sex skills to beat their raw power, she then evolves during the sex like you said.
I think the MC getting protection from Necrozma after banging Nebby/Alt-Universe Nebby had already been suggested, the three of them fusing back into Ultra Necrozma for some fun is good though!
I can make the Darkrai/Manaphy scene work, I doubt it will make her and Lugia will agree afterwards, but it could still be fun!