ı would like to say "military non-solution" was meant as a very long period of action that would have sapped the resources of the British Empire , decades of a guerilla war style . Even if massacre as a population control was much more acceptable back then .
It's questionable whether guerrilla war would actually have dislodged the British without political or conventional military pressure being brought to bear elsewhere. Irregular warfare does not tend to be an effective way of achieving a lasting political settlement unless some political or conventional military force is brought to bear against the occupier. Most of the famous guerrilla conflicts of the twentieth century were ultimately won by conventional forces- China, Vietnam- or because political pressure forced the occupying power to withdraw- Ireland, Algeria- rather than because guerrilla activity was sufficient to simply collapse the military effectiveness of the occupier. While political pressure could have forced British concessions in North America, they would have stopped well short of independence, which even among British opponents of the war was regarded as a dangerous gamble. An independent United States of America- even one nominally under British sovereignty, a "Dominion of America"- was only ever going to be on the cards if the British military was broken in North America.
I'd also quibble the acceptability of massacres- one of the major events leading into the Revolutionary War was the Boston Massacre, which was so controversial that the British were forced to allow their own soldiers to placed on trial in a civilian, colonial court- and to allow that most of them were convicted. It was different if the victims were imagined to be racial inferiors- Indian or, y'know, Irish- but the killing of British subjects by British soldiers was widely agreed to be beyond the pale. Massacres of civilians did occur during the Revolutionary War, but almost all instances were carried by American irregular forces, whether Patriot or Loyalist, and represents the particularly vicious nature of the civil war in the more remotes parts of British North America, rather than the widespread acceptance of civilian massacres as a tool of public order.
as for the Spanish connection and suspectability of the "then" Americans to play a Spanish game , ı would surely invoke stuff that the rebellion would indeed end very quickly or some sort of a crusade to be assembled to attack those "Spanish lovers" if they were to fall into such "misguided" paths . British patriotism of Americans show up regularly , like this Monroe doctrine stopping the Spanish really working on South America so that British business interests could fund a continental uprising for profits and stuff .
The Americans had no particular animosity towards the Spanish, moreso that than they did foreigners generally. Their traditional enemies were the French, and the Continental Congress actively sought French support during the Revolutionary War, so even that did not prove to be an insurmountable grudge. Moreover, hostility towards the French and Spanish was very generally framed in the terms of hostility to Catholicism and to absolute monarchy, but it would very difficult to present Calvinist republics as somehow more closely-situated to Roman despotism than the royalist and high church Episcopalian government in London. It would be very hard to frame any such war as a "crusade", to disguise its true nature as the pacification of rebellious border provinces, and consequently very difficult to rally public support, in America or Britain, for what would be an arduous and expensive campaign of little obvious commercial or strategic value.
Moreover, there is the simple
how of it. The British had already struggled to impose their authority over the Appalachians before the Revolutionary War; the tensions that arose from this were one of the major factors leading into the war. The last British military presence in the region was in what's now Detroit, far from the centre of settlements along the Ohio and Tenseness rivers. The important strategic positions in the settled region were either controlled by American rebels, or by Indians nations who were at best
aligned with the British. Even if the British could have carried out expeditions against these positions, it's unlikely that they would have, or could have, spared the men and resources to hold the surrounding territory in any enduring way. Even when the British took and held Fort Pitt during the French & Indian War, the assumption was that it would serve as a deterrent against the Indian tribes in what's now Ohio, and offer some measure of control and security for the settlers in Western Pennsylvania, not that it could be used to projected power down the length of the Ohio Valley. When the rebels you are setting out to crush live in a series of fortified villages scattered across broken and forested landscape, where do you land the sledgehammer blow of a major conventional military expedition?
It's more likely that you would see a number of smaller expeditions, with contained goals- occupying specific points, retaliating against specific offences- with at most the hope of convincing the Overmountain secessionists to voluntary return to the imperial fold.
