>>750I'm not sure the ink smudge theory holds because writing predates ink by millennia. The earliest examples of what we would consider fully formed writing is stone tables (made from stone that naturally splits into flat slabs like shalestone) and clay tables. An the writing was done by scratching the material. Most of the stone and clay table already had the common directional writing conventions that are still in use.
On top of that, abrasion writing instruments using various solid consumable materials that worked like pencils or chalk were more common than liquid ink, because ink was very expensive. Ink used to be based on pigments suspended in oil and solvent, which took hours to dry. So it would smudge no matter what. People that wrote in ink used tables with steeply inclined surfaces to aid in ink-touching-avoidance and they dried the inked paper like laundry.
However you are probably correct that right handedness was the reason for the left-to-right writing direction. Just because that way your hand would not obstruct your view while writing.