Regarding #3, tax withholding is, by default, treated as being paid evenly throughout the year. However, you have the option to treat it as withheld when actually withheld if using Schedule AI to annualize. That could be helpful if you had a larger amount withheld at the beginning of the year, but could be detrimental if you had a larger amount withheld toward the end of the year. In your case with your only withholding occurring near the end of the year, you would want to use the default treatment for tax withholding.
