Authentic image of a very historic moment?

Guillaumot-Richard, from France, sells on 26 May 2018 a "Gouache, dated 1785" depicting the "Aéro montgolfiere de Pilâtre de Rozier et Romai : la traversée de la Manche" or "Aero-balloon of Pilâtre de Rozier and Romain: the crossing of the Channed", estimated at 400 to 500 Euro.

This small (17.5 by 11.5 cm)  gouache depicts what looks to be one of the glorious moments of the very early ballooning years (the first crossing of the Channel from France to England), but turned out to be the first really dramatic moment in that history, with the first fatalities of an air crash ever.

Jean-François Pilâtre de Rozier was actually one of the two men in the first real, untethered ballon flight in 1783, and also flew on the second balloon flight. He continued pushing the boundaries and went on ever more daring flights, going as high as 3,000 metres in 1784 already. 

The first Channel crossing, from England to France, had alreday been made early in 1785, but De Rozier wanted to make the first crossing from France and developed a new type of balloon for it, called the "Aëro-montgolfière" (Montgolfier was the inventor of the hot air balloon, and all balloons were called montgolfières then). It not only used hot air but also hydrogen, which has a larger lifting capacity. 

His companion Pierre-Ange Romain is less well-known, but was also a physician and chemist and co-designer of the new balloon.They died when the wind suddenly changed and the ballon imploded (though it didn't catch fire, it crash-landed, killing both passengers).

The work has the year 1785 and what look to be initials at the front, and an explanatory note on the back. It reads 'Aëro-montgolfière of Pilâtre de Rozier and Romain, in which they perished 15 June 1785, while flying across the Channel. A Wimille Pas-de-Calais". 

Pas-de-Calais is the region in the north of France where this happened. Their starting place was Boulogne-sur-Mer and the crash is usually situated in Wimereux. The actual crash took place in Wimille, in a part of the town which is now part of Wimereux; so the text at the back at least has the historical details completely correct. 

The problem with this kind of historical artwork is knowing whether it is an original contemporary eye witness account, or a (much) later commemorative, invented image. If this is an invented image, it is probably based on the above engraving by Charles Echard, showing clearly the same balloon and tower, but from a different vantage point. 

But I think that this one may well be a real contemporary document, an eye-witness account, and as such it should be worth a lot more than the estimated 400 Euro.