If my tri-con character with a naval enhancement on it's military icon is knelt, can I use it for a power challenge?

I am not asking if I can naval it in. Just a normal power challenge.

Yup, the Battle for Shield Islands allows any character with a naval enhancement to bypass the requirement for kneeling them to participate. That said, they still have to have the correct icon (so your tricon is fine, but a monocon with a naval enhancement can only participate in challenges for which he has the icon).