What I found odd in reading the account was how Donaldson seemed to come upon the apartment almost causally. Then, given the form of the discovery, "when the storekeeper looking for a place to put them, when we joined him."
Now, without "clearing" the French from the charge, that formulation suggests others may have been about. Consider he said earlier "the inhabitants had mostly left" and the sequence of the discovery was after, "We had scarely taken up our quarters".
In the Chapel there was a "large fire in the middle of the floor" and various items broken including the "organ broken up for the purpose". What purpose?
Then the apartment was dark (Donaldson had to go back to get "a buring piece of wood") although embers seem to have still existed. That suggests the apartment fire was long past.
Given the darkness of the apartment (and my mystery-show mindset), there is the possiblity the people hid there (other than the strangled infant - why in a sack, though?), were closed in (by whomever vandalized the rest - disciplined French, laggard French, brigands, or - even - advance "elements" of the British) before a fire occurred (it seems to have been inside the apartment and not outside, making it difficult to see how people outside could set the fire - outside there is still plenty of wood around for burning).
So, when, how nor why were the people in the apartment? Who set the fire?
Given the time line, it is possible that all manner of unsavory characters may have been involved.
As to numbers, Edward James Coss's disseration "All for the King's Shilling" mentions the 200 victims and references Donaldson, yet the page reference was 210 and, as mentioned before, the pages are 107 to 109 for the version of Donaldson available to me (published in Philadelphia in 1845).
To be clear, my intent is to understand and, thus, these are questions not assertions.
Any help would be appreciated. - R