Lord Wellington assumes the Spanish throne

Let's give the scenario a chance shall we.

In my mini-Tl; Wellesley encounters greater success in the Peninsular War, winning the support of the Spanish generals and through well publicised actions of compassion and mercy he gains a very large spread of popularity within the Spanish populace. In response to Wellesley's greater successes; Napoleon sends greater amounts of men and resources to support Joseph but he still launches his campaign in Russia. A major POD is that Ferdinand of Spain dies during his imprisonment, rumoured to an assassination; that this soon followed by the disappearance of his brothers Carl and Francisco trigger even more uprisings against the French regime in Spain; as the 'dead' royals become martyrs for the growing guerrilla forces. However Coalition forces are now paralysed with indecision and are unable to seize the advantage due to the infighting within the ranks of the Spanish junta as the are unable to agree upon who should take the reigns as regent or whether they should select a new monarch from the Bourbons dynasty or even take up a new dynasty entirely; ideas of republicanism are quickly disregarded by the conservative generals at the annoyance of the younger and more liberal junior officers.

At certain points the Coalition flirts with catastrophe as various Spanish generals make their case for why they should take the crown arguing on the basis of wealth, their family names, the number of troops in their personal armies. Tempers rise and a Spanish civil war is only narrowly averted by the request of Sir Arthur for an evening break and reconvening in the morning; letting hot tempers cool and nurse their bruised egos. It takes a month and a half of negotiations, bargaining, begging and implied threats but a eventually there is a conclusion. Taking example from the Portuguese who made a British general their Commander-in-Chief; recognising Sir Wellesley's military victories and keenly aware how deeply reliant the Spanish economy was upon British credit, They requested that Sir Wellesley take up the position of regent for the duration of their conflict with France and thus recognise him as the Duke of Ciudad Rodrigo; and tentatively agree that it should be a Cortes of All Spain that should decide on the matter crown not merely the army.

There's something of a lull in the conflict as the Parliament in London digests the news that one of their generals is now the Regent of the Spanish Empire with all the powers and responsibilities that such a title implies, at the same time Joseph Bonaparte escalates large scale liquidations of Napoleonic Spain to pay for mercenaries from Germany, Switzerland, and Italy in addition to requesting more conscripts from France to maintain some level of control over the country and planning a number of small counteroffensives to try and regain the initiative in the war. At the same time the Duke of Ciudad Rodrigo reorganises the Coalition forces, incorporating various guerrilla armies and private militias into the Spanish Army and managing the various egos and rivalries within the senior officers of the Army and the Navy.