What is the earliest point at which naval technology was sufficient to make a crossing of the atlantic from mainland europe to the americas, barring the northern route the Vikings took? I know it happened in the 1400s but could it have happened in say the 1200s or even earlier if someone was crazy enough to risk it?
The Americas were discovered in the 10th century (Norse ships left Iceland for Greenland in 985, and the Newfoundland Norse settlements date to about 1000). The technology to discover the Americas existed then; that's when the Americas were discovered. Excluding the northern route the Norse took is nonsensical, and I'm curious why this is a criterion. (This is of course leaving aside the issue that the Europeans didn't "discover" America.)
I've answered a similar question before, which you can see here. If you really want to argue that the Norse could have loaded a knarr with five weeks rations and set off from the Canaries in 985 (they could have), I won't stop you, but it gets into the realm of the hypothetical.