while the idea isn't bad, what's the reason to cancel a naval invasion? canceling a naval invasion basically means you retreat your ships, and that equals failing an invasion, so there's no point in having that as a feature. you can just remove your task forces from the invasion zone and the invasion will fail within one or two turns if you have no ships in the zone anymore.
since the players decide where they want to invade, they should be able to know when they don't have enough ships available for another invasion