Yes, "mid-forties" should be hyphenated. The hyphen helps clarify that "mid" modifies "forties" as a single concept, indicating the range of ages from approximately 43 to 47. Without the hyphen, the meaning could be less clear.
Can you, or should you? You can hyphenate it if you're moving between lines in a paragraph and need to break up the word. You shouldn't hyphenate it normally.
Yes it should be hyphenated.
not required
McCarthy should not be hyphenated.
A hyphen is not required.
Yes it should be hyphenated: co-worker.
Yes, hyphenate by-laws.
Yes it should be hyphenated.
According to the sources I've found on a quick search, you should hyphenate 'coauthor', i.e., it would better be: "co-author".
Don't hyphenate; ongoing is one word.
You do not hyphenate the number.
I think it's more accepted to hyphenate it.