I don't get, if Peru becomes a monarchy during the time of Bolivar and San Martin, who becomes king?
Sorry if I wasn't clear. I was talking about two different things. I meant to say that I could think of two possible scenarios in order to create a huge Spanish speaking South American state: one
with a Pod after 1810, which has San Martin and/or Bolivar as main characters (as in your original post), and another
one with a Pod in 1806/7, which has the Spanish Kings as main actors (an alternate scenario). In the second case, the King would be a member of the spanish royal house who would have escaped to peru in order to flee from Napoleon.
In the first case, having a huge Gran Colombia, stretching from Venezuela to Argentina is extremly difficult, for the reasons posted above. On the one hand, geography conspires against such a state. On the other, this state would have had a core in the distant North (if Bolivar founds it) or in the far South (if San Martin founds it), and this would have made even harder to keep such an "unbalanced" state united.*
In the second scenario, Perú is the core of the new state, and its rulers, the Kings have a "traditional" letimacy that Bolivar and San Martin didn'thave (they have
another type of legitimacy, of course). In this case, mantaining such a nation united is still difficult, but I think it's easier than in the previous case. This nation would probably lose territory in the North and in the South. And it wouldn't be exactly a Gran Colombia. But it's a scenario which gives you a huge Spanish speaking state in South America.