You can fabricate claims across one sea zone if independent, so no, you don't need a land border. You just need to not be implausibly far away.
I'm not sure why a viking invasion wouldn't work (though it may be due to you being too small). It's not at all limited by any range factors. Also, pagan_county_conquest would work, because that too has no range limit and can be done anywhere coastal. It requires a little piety.
You do need 50 prestige and 10 piety for the Declare War button to even be available, though.
EDIT: Also, the Become King ambition just allows you to do unlimited Pagan Subjugation wars against rulers within your de jure capital kingdom, Norway. However, you do not border Norway, which is why Pagan Subjugation is not possible. Pagan County Conquest against anywhere coastal and Christian should be fine, though. [Yes, even Constantinople from there.]