Any Russians present at Merxem 1814?

@Eltjo Verweij I'd not mentioned the sailors in my initial answer since they were clearly not at any of the actions that you were interested in but you are quite correct. British accounts make no mention of cossacks after Benckendorff left the theatre - if any were involved in the operations against Antwerp, they would have had to have been away to the south, on the left flank of the Prussians, so certainly not at 1st or 2nd Merxem.

@Andrew Bamford You were right, it was at Hoogstraten. Found it in "einem preussischen Officier" (1843) 'General Graf Bülow von Dennewitz in den Feldzügen von 1813 und 1814' p. 282 and in Greg Gorsuch's translation of Maurice Weil (1891) 'La Campagne de 1814':

"The 11th at 8 o'clock in the morning, Borstel, with the left, began the attack on the post at Hoogstraten, where General Roguet, whose vigilance had been alarmed by the large number and boldness of the parties Cossacks was preparing for a reconnaissance with the brigade of Flamand."

In German it sounds like this: "Am 11. Januar griff General von Borstell um die achte Morgenstunde Vortet und Hoogstraten an. General Roguet, durch das frühe Erscheinen einiger Kosacken-Abtheilungen zeitig aufmerksam gemacht ..."

So which cossacks were these? It seems there were at least several sotnyas. My guess is that they were the Bihalof I Regiment, which had cooperated with Borstell on 24 December 1813 in the attack on Wesel, and were destined to support the Saxon Corps in February 1814.

@Eltjo Verweij Great stuff - always useful to tie up these details.