Complex sentence: Experts at the National Portrait Gallery (and elsewhere) have disputed this, suggesting that the painting dates to the early 19th century and thus cannot be of Austen, or painted by Humphry.
Split into simpler sentences: Experts at the National Portrait Gallery (and elsewhere) have disputed this, suggesting that the painting dates to the early 19th century. If the portrait is of this date, it cannot be of Austen, or painted by Humphry.