Yeah, the more I think about it, the more impossible it sounds. Preston was wholly a land engagement. It's not (totally) ridiculous that a Maryland seaman would head back to Scotland to support the Royalists, though the Civil War broke out in 1642, so you would think if he was so committed to it, he wouldn't have waited until 1648 to go back and help out. But why is he in a land battle? The Royalists had plenty of naval action going on. Why wouldn't he be there instead of thrusting pikes with a bunch of Scottish levies under a gaggle of idiot commanders?
And then there's the transportation. Mackenzie supposes that he must have escaped from his master, which is indeed about the only possible explanation, since all the Scots transportees seem to have been sold for 7 years. But in 1650, he supposedly gets a land grant for headrights-how does a guy who's on the lam (A) get enough money to transport four people and (B) keep himself out of irons when he shows up in Williamsburg with his claim? Indentured servants in the 1640s were notoriously ill treated, and you can imagine what they did with the runaways. The guy's basically an outlaw, but supposedly he's collecting a 200 acre land grant? It's still 1650 - the Commonwealth runs Virginia as much as they do England. Maybe I could believe that after the Restoration, he can run around doing all this without an alias, but the story sounds highly implausible in its historical context.