Question 1 relates to automatic visa re-validation process and based on the facts you provided you should be able to do that if the trip is fewer than 30 days.
Question 2 is unclear if you mean whether the travel will affect a pending extension petition or if the extension petition is submitted after your return from Canada. If the latter I don't see any facts in your question that would adversely affect the extension petition.
Question 3 -- this is a very open ended question; of course there are the basic travel documents and documents that verify you are still working for the H-1B employer. Beyond that I am not sure what you mean by "re-enter USA safely."
Finally you should discuss any particular specifics in your situation with your own attorney or your employer's attorney so that you do not jeopardize your employer's H-1B extension petition.
Your question is way to detailed for the online forum like AVVO. Talk to an immigration attorney who is handling the case for the employer. In general, to return from a 30 day or shorter trip in Canada or Mexico a nonimmigrant with a valid I-94 who maintains the status in the USA, one only need that I-94 and valid foreign passport.
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