It is likely that the Grande Armee were in possession of Russian pieces at Borodino but where did they get the horses to pull them. They were left upon the field because the Russians could not withdraw them. As far as I was aware Borodino was a bloody draw with the Russians leaving the field of corpses to the French. There was no pursuit and the Russians gave up Moscow without a fight.
The only guns that arrived back in Poland and Germany were from the Northern and Southern sectors.
If the Unicorn was so prized then why was the Russian gunnery so poor. Now I hear you go through the training etc...I have not read any independent sourse that says this. It was doing a different tactical job to that of the French. There were issues of the gunners pre-1807 firing at too long a range as they were tied too closely to the infantry. Also I remember the statement that Napoleon learnt from the Russians about concentrating artillery at Eylau.
The Unicorn was so prized because it was longer calibre and hence more accurate. It had twice the calibres to the Gribeauval 6.4in which had not seen service since 1800s. Those in Spain were Spanish. The M1795 Long Porte and AnXI 24-pdr were 7 calibres long compared to the Gribeauval 6.4in of 4 calibres. The Unicorn was 10 calibres.
Although I can see why it was probably used as the Grande Armee expended most of its ammunition and supply was a problem no doubt. The logistic side of this war has been poorly supported with glimses in Digby Smith's books.