The easiest way to make sure you do this the right way is to mentally remove the entire parenthetical portion. Then ask yourself, is the sentence still punctuated correctly?
In this case, we have a problem (because I've put the period inside the parentheses.)
In this example, everything is fine (because the period is still at the end of the sentence).
(This one is very awkward, because the exclamation point is left all by itself)!
(A parenthetical statement without any other text should include the period inside the parentheses.)
The first and third sentences are incorrectly punctuated, for the reasons explained in the sentences themselves.
If you do use a period, it always goes after what is being abbreviated
Dr. (doctor, not to be confused with "drive")
St. (saint, not to be confused with "street")
Ph.D.
However, a period is not always used, especially in BrE, and seems to get dropped from time to time in AmE, for example in addresses.
In your first example, putting the period before the "C" would result in
John .C
since the period is usually not a word delimiter in written English, otherwise it would be
John C.Williams
which would be incorrect, however, there are times it might be formatted as
John C.J. Williams
for contiguous abbreviations.
The famous example which breaks the rules is
Will.I.Am
Formatting the abbreviation as
John.C
looks more programming-ish, "C" is a method for object "John".
Best Answer
This one is fairly easy, I went to Google. On the envelope after the city some use a comma and some use no punctuation.
In the body of the letter:
LINK
I have never seen a full stop after the writer's name.