The issue of Spain was just a little more complicated than that. Spain was an ally until Napoleon found evidence in Berlin that Spain was prepared to turn on him if the Prussians won. As he was in the middle of a war with the Russians, Napoleon could not deal with Spain directly in late 1806, so he required his ally, provided for in the Franco-Spanish treaty of alliance, to supply an auxiliary corps which was under Brune’s command against the Swedes. When they mutinied under Romana and over half (8,500) were taken away by the Royal Navy, about 5,000 didn’t get out and many of the Spanish were pro-French, including Romana’s second in command, Juan de Kindelan.
So, Romana’s corps wasn’t ‘lured’ northward and they served ably at the siege of Stralsund. The majority mutinied after Charles IV was dethroned (with the help of his son, Ferdinand).
The Spanish also agreed to take part in the invasion of Portugal in 1808, a fact that is usually overlooked. They were not innocent on the international 'stage.'
The French did institute needed reforms into Spain under Joseph to the benefit of the Spanish people, and it is noteworthy that both Soult, in Andalusia, and Suchet in northeastern Spain, pacified their areas of responsibility during the French occupation. And it is also noteworthy that Ferdinand, upon his return in 1814, treated those that had fought for him badly and dishonorably. And it should also be noted that more money poured into Spain from France than the opposite from 1808-1814. Ferdinand was both oppressive and arbitrary, much more than either Joseph or Napoleon.