Does my saying "as they please" – upon hearing someone backbite me – count as forgiveness or as condoning backbiting, and should I turn away from this line of thinking?

There is no need to ponder this statement. You should avert yourself from this type of thinking so that it does not lead to obsessive doubts (waswas). This statement does not make the unlawful lawful; rather, it signifies indifference to what is said about you, or it falls under forgiving them, which is considered a virtue and a means of drawing closer to Allah the Almighty, as corroborated by His statement: {And that you pardon is nearer to righteousness}, and the Hadith: "Connect with whoever cuts you off, give to whoever deprives you, and forgive whoever wrongs you." Some Companions even offered their honor to those who wronged them.