PEI was already a separate colony by 1777 although with a population of around 2,000 while as you stated New Brunswick was still part of Nova Scotia.
Didn't know about PEI < St. John's Island being a separate colony in 1777. Makes sense.
We had a separate thread a little while ago about what implications of micro colony PEI would have on the constitution talks. Some argued that it would be become part of Nova Scotia but I argued that fly against the idea of each of the 13 original colonies principle of self determination. If PEI could be forced to re-join Nova Scotia what to stop the southern states from demanding sane with RI or other smaller norther states.
Because Rhode Island is made up of a bunch of stubborn people who prefer to beat to a different drum, giving everyone else - North and South - a permanent headache.
With regards to Nova Scotia (and hence the question of PEI), the question becomes whether or not the New England militias and the Continental Navy/Army are able to get past the strong British fortifications in Halifax and thus gain control over Nova Scotia, which I severely doubt. If the Americans gained control over St. John's Island, regardless of the circumstances, it would remain a separate unit.