Solitude Hold guards are trained by the Legion, Captain Aldis is a Captain in the Imperial Legion and in command of the Solitude guards, the guards outside of Solitude in the Hold are a part of the Imperial Legion faction. All are Nords. There are no Breton farmers, the only two farmers that move to Civil war factions is a Dunmer, Imperial.
Many of the 'Imperial Soldier' located guarding Castle Dour are Nords. Most of the Imperial Legions in Skyrim are Nords, I've also come across a Redguard Quartermaster in one of the Camps.
Taking over forts, there is a large number of Nords and Imperials, some Fort Commanders are Nord.
Nord Legionnaire, Fort Commander.
After seven long, bloody years the Stormcrown Interregnum was ended when a Colovian warlord by the name of Titus Mede seized the crown. Whether he had rightful claim or not is moot. Without Titus Mede, there would not be an Empire today. He proved a shrewd and capable leader, such that Skyrim endorsed him as Emperor. With the Empire stabilizing under the auspicious efforts Emperor Titus Mede - Rising Threat, Vol. IV
Usurping Ulfric isn't easy, a province full of Stormcloaks who are loyal only to him will stop you. Having the dragonblood hasn't stopped Nords in the past taking up arms against it. Battle of Sancre Tor, War of the Red Diamond etc.
Well as High King your word won't mean much, the Holds of Skyrim are all largely independent. How magic works is you need to have some skill in it, a natural ability. The player has it because they're the player. Not every Nord can practice magic.
The Way of the Voice forbids using the Thu'um for violence or personal gain unless it times of true need, expanding borders isn't true need. Every person who used the Thu'um for their personal conquest ended with destruction. Pissing off the Goddess Kyne is not something you want to do. The last Nord army that had been using the thu'um for conquest they were annihilated and the way of the voice was born, realizing their error, that it should only be used for enlightenment and worship of the Gods. The Dragonborn may be an exception to the rule to an extent, but Talos had his throat slit and could never use the Thu'um again. Every other recorded Thu'um user who used it for violence it did not end well. Chances are Kyne will smite the Stormcloaks and Ulfric.