The British retreated from Boston after an 11-months-long campaign in 1776, true?
And the British took New York City after a 2-3 months campaign (depending on whether one considers Harlem Heights the last battle of the New York campaign or the first of the New Jersey-Pennsylvania campaign), so the historical record seems pretty clear neither the British nor the Americans were "
undefeatable" on the battlefield in this period.
The British and Americans also swapped victories in Lower Canada in 1775-76, so there's another point of comparison.
What impact the tactical element has on the operational, much less strategic, is much less clear, given that the same "defeated" Continental Army in 1776 was still able to "defeat" British forces at Trenton, Princeton, and Saratoga, absent a Frenchman or Spaniard...
Saratoga, of course, was in 1777, and the Franco-American alliance was not signed until 1778, so the 1776 limit on "the French coming in" seems a little arbitrary.
Given that the OP was "Could Britain have won the Southern Theater of Ammerican Revolutionary War" with an end state described simply as "decisive victory" (undefined) and "whether the British could have won in the south and how victory could have been achieved." (also undefined) pointing out the
strategic and operational problems the British (like all the other European powers) had in any attempt to achieve a "decisive victory" in the Western Hemisphere against their rebelling brethren hardly seems worth an epithet, does it?
Best,