You are making a confusion between multiplayer and strategical flexibility or liberty I am afraid
Nothing in the game prevents you from concentrating on Spain, or on the contrary abandon it altogether and play a XVIIIth dynastical conflict in the way of a WW2 American strategist focusing on the main enemy first... you'll just not be rewarded for it, but it is entirely feasible...
Marlborough can go wherever you see fit, or Eugene, or anyone else for that matter... and same for the Bourbon side
The style of game you want if perfectly possible as the game is now