It depends upon when your hard credit report was pulled. Most lenders require a mortgage to close within 120 days of the credit report being pulled for that report to be used.
Yes, you can always cancel a refinance application up until closing. A cancellation might result in paying third-party fees, but the right to cancel is available. In certain situations, you can cancel your refinance application up until 3 days after closing.
Yes, a borrower can switch from self-employed to W2 job and get qualified for a mortgage. How long the borrower will need to be at the new W2 job will depend upon the type of W2 income they get (salaried vs hourly vs commission).