Of course, the goal at first was getting to the Indies. The Spanish might have been distracted on the way, if Panama was open, by Inca gold. But the big fight would then be who controls the opening and route. I suspect North America might have been an afterthought.
“Buenos Dias! We have the mountain of gold here!”
“Bon jour! We 'ave le beaver. Plenty of…”
But had poutine been invented a few centuries earlier, all the colonial action would have been up North. 
Not really. In fact, that was more-or-less the situation at times in the past. Not just because of tectonics, but also because sea level may have been higher - continental crust doesn’t have to mean dry land.
Continents do, indeed, form from the accretion of smaller bits. Large parts of the interiors of the current continents are larger stable bits (cratons) but there are plenty of microplates as well, and not just in the oceans. That, at least, is the current thinking (and by “current”, I mean in work published this year.)
You could postulate the existence of a continent where North America is, but one that is low-lying, so that only the tops of mountains of that continent are above sea level and form a massive archipelago. Kind of like New Zealand is a high-lying plateau of a submerged continent.
