Dear native speakers (this time, especially, U.S., Canada, and U.K.),

I have long been confronting the problem related to the usage of commas and periods in quotation marks.  I know that periods and commas are placed inside quotation marks in U.S. style; but they are not in British style.

Example)
U.S. style) I said, "I cannot do this."
British style) I said, "I cannot do this".

Here are my questions:
1. Is it really true?
2. Do British/Canadian people use double quotation marks in such a case, not single quotation marks?  Or does it depend on the writer (which to use, single or double quotation marks)?
3. Is it true that, in American style, commas and periods are placed outside quotation marks if the word enclosed in quotation marks is only one letter or a number?
e.g., This is "1", not "10".
e.g., This is called "X".

Someone, please, please help me clarify these points: