Your vision is always a singular dot in space, unless you have amblyopia or something else causing your eyes to point in different directions. Even if you’re looking at a featureless line, your eyes are fixed on a single point in space. It’s not like one eye is looking somewhere different from the other eye. The triangle still exists.

There is no difference to stereo vision depth perception regardless of how the view points are oriented in the X and Y axis. The practical proof for this is in rangefinders; neither simple consumer stereo rangefinders nor complicated military stereo rangefinders, such as those found on battleships or antiaircraft guns, are oriented in the Y axis to gain some advantage. There’s no need. The triangle is the same regardless of X-Y orientation.

I see what you’re saying about the example of two completely featureless lines oriented exactly along the X axis in a completely featureless space, as this wouldn’t work with a coincidence rangefinder, but this is an edge case and not something you’d encounter in real life. You’d also have other cues for depth perception beyond just stereopsis.