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I’m gonna double down on the Mexican American War and say that they are US Volunteers. They have the correct kit, the caps are fitting of the American army at the time and the inclusion of the slouch hat makes me think that they are an irregular/volunteer unit. While that is my best guess, I still am not 100% sure. I’m dying to know for certain.
Now I am with you and over on the American continents. Could work for north, central or south, I reckon. They look particularly 'irregular' with that smock coat on the 'shooter'. If it weren't for the previous officer I'd have gone for early 19th C. I seem to recall a picture in Allen's 'Fighting Men and Their Uniforms' of a Mexican irregular infantryman from the Mexican-US war shown in a similar habit and kit. I'll have to wander over to my shed to check...
Regardless, there is plenty of flexibility with such figs.
Lastly, and most importantly, it is so absolutely marvellous to see Strelets keep on keeping on and extra grand that you are providing us with the joy and challenge of a Friday teaser. Thank you!