Leonhart's plan was very simple. Actually if it was a throne succession of a human kingdom then it would have been more complex, but beastkins were simple species. If you are the strongest among the other princes and have the strongest support behind you then you can become the king easily.
If the other princes are not accepting then defeat them, if the king is not accepting defeat him too. And then you would see that everyone would start to support you. So Leonhart's plan was to simply strengthen himself using several dungeons that the marquis had in his lands and also to win over other strong nobles.
Having the support of a marquis was definitely good but only it was not enough. He needed more noble's support if he wanted to get the throne without any uncertainty.
