Yes, "previously scheduled matter" is hyphenated when used as a compound adjective before a noun (e.g., "The previously scheduled matter was addressed during the meeting").
Hyphenating is a matter of choice and what makes a sentence clearer; rules about hyphens leave some room for a writer's own judgment. For example, the question, 'Is one half hyphenated?" could be read as, 'Is one-half hyphenated?", or 'Is one half-hyphenated....' (A half-hyphenated what?). It is an oversimplified example, but it doesn't take much to confuse. Another example, 'English language learners...'; is this people from England learning a language or learners of the English language. The use of a hyphen, makes it clear, 'English-language learners...'The purpose of hyphenating is to overcome ambiguity.
Yes, "self-diagnose" is hyphenated.
No, "self-interest" is typically not hyphenated.
"Left-handed" is a hyphenated compound word.
The word "self-confidence" is hyphenated.
It can be one word "reelecting" (previously many such words were always hyphenated, and some still are).
yes
It is not hyphenated.
The idiomatic usage is not hyphenated. The term "used to" is a colloquial phrase meaning either - (adjective) accustomed to - (auxiliary verb) did previously, in the past
No it's not hyphenated.
Motorcycle is not hyphenated
words are only hyphenated when they have a separate meaning when separated than they do when hyphenated
No, "hardworking" is not hyphenated.
Yes it should be hyphenated.
Ossification
It can be. Some words can be seen both hyphenated and non-hyphenated.
Yes it should be hyphenated.