On the later problem - later series becoming dump and go - I think that's true of most of our sealed formats. King Block is worse - there are two Nazgul plus two site support cards, and Enquea is better than
Southron Commander. There's not many anti-companion cards in Shadows block either.
I think knowing how to play
Southron Commander is the most skill-intensive part of this format - and why I win a lot of games in Series 3 and 4. I see so many play errors, like playing out both Commanders at the same time, or not leaving 2 pool to at least bluff a pump card. I don't want to remove one of the few skill decisions in the format.
Your proposed minion is overpowered. 13/3 for 5 against 4 companions is too much. So is 13/3 fierce D+1 against 5 companions. I would move the +3 to 5 companions, and the fierce D+1 to 6 companions. And I'll give it a new name.
Easterling Shadow
Minion * Man * Raider
Strength: 10
Vitality: 3
Site Number: 4
Easterling.
While you can spot 5 companions, this minion is strength +3.
While you can spot 6 companions, this minion is fierce and damage +1.
Assignment: Spot 6 companions to assign this minion to a companion (except the Ring-bearer).
I don't think this solves the problem, though - it just changes it from going to the Ring-bearer to killing other companions. And it's worse in the late game.
Southron Commander can stop a fellowship at site 8 and kill at
Palantir Chamber - Easterling Shadow is not going to do that. It increases luck, because I need
the Easterling Shadow to do damage earlier.