Complex sentence: Whatever the truth of the matter, the "Black Paintings" murals probably date from 1820 and were likely finished no later than 1823 when Goya, departing for Bordeaux, left the villa to his grandson Mariano, Arnaiz (1996), page 19.
Split into simpler sentences: Whatever the truth of the matter the "Black Paintings" murals probably date from 1820. Most probably the murals were finished no later than 1823, when Goya departed for Bordeaux and left the villa to his grandson Mariano, Arnaiz (1996), page 19.