Conjunctive adverbs such as "however" and "therefore," and transitional expressions such as "for an example" and "in addition" can express connections within sentences. When they do, set them off with commas.
Therefore, the American Midwest is considered the world’s breadbasket.
California and Florida, for example, are important food producers.
COMMA CAUTION: Use a semicolon or a period-not a comma-before the conjunctive adverb or a transitional expression that falls between independent clauses. If you use a comma, you will create the error known as a comma splice.
Therefore, I refuse to go... (three dots) This is correct because it shows a connection to the previous sentence.
And so, he did it…
I am not sure, but I think "and so" in this case is used to show a connection to the previous sentence like a conjunctive adverb, so it might be separated with a comma. Actually I think it is a misused comma because both "and" and "so" are conjunctions. My grammar checker shows that either way is correct, but grammar checkers are some times incorrect. Lets post this question and see what others have to say about it.
All I can find is avoid misuse of a comma with subordinating conjunctions and prepositions; do not put a comma after a subordinating conjunction unless another rule makes it necessary. I cannot find a rule that would make it necessary to use a comma after "and so."
One question mark is correct, and two are over used.