To answer your first concern, hand initialing and dating the stamp with ink (later with a device) had the effect of cancelling the stamp; acknowledging payment--and in compliance with the law.
One onliner expressed this: "Faced with the financial demands of the Civil War, our Federal government issued the Revenue Act of 1862. This Act created ways to raise new revenue, as well as formed the Department of Internal Revenue. Nearly every kind of document was taxed (including deeds, insurance policies, telegrams, stock certificates, etc) and what were considered luxuries (such as playing cards, liquor, tobacco, matches, perfume.) Revenue stamps were designed to affix to these various items proving that the tax had been paid."
Don......