Vaumel, for instance, need only rule up Seaward high enough (L7 I'd say) so that he can upgrade his shipyards and start cranking out a few galleons. As he isn't lacking for money nor the ability to decieve Avan, the only real obstacle is his ability to actually rule a province that high. Once he did all of this, well...then it would be high time to declare independence from the oppression of Avanil! And at that point he could style himself just about whatever he darn well pleased. Mieres is all about naval supremacy, so it really requires him to get that navy into a position of dominating the straits and defending against anything Avan might launch against him. Not to mention the overall power Vaumel could wield by dominating trade (and piracy) in the Straits...Mieres has some awesome potential to be a dominant power in the south, though the only real lacking quality is probably Vaumel's incompetence as a landed regent and his short-sighted greed blinding him to the big picture. Nevertheless, as a PC realm where a better regent comes into power, it has some great potential.
And isn't this exactly why Darien Avan would never let any ruler of Mieres develop a level 7 province or a shipyard of that size. The threat to Avanil could be catastophic. Their is no real way a province or shipyard of that size could be developed without it coming to Avanils attention. If I was Darien I would have put out a decree a while back setting a cap on the development of any single province within Mieres beyond 5 and would inforce it with brute force if neccessary. I would also forbid Mieres from buying to strong of a navy, building a fleet to be rented out to Mieres if neccesary, but making sure that the admiral was a trusted lieutenant if not a close relation.