Is saying: "Forgive me for my words and deeds" considered absolution from backbiting without mentioning it? And is it obligatory to make up for someone who doubts the fasting of days they missed in Ramadan, with the strong presumption that they made them up?

As for repentance from backbiting, a general absolution is sufficient without specifying the right. As for one who doubts about making up missed fasts, the principle is that their responsibility remains until they attain certainty of absolution, and they must fast what they are in doubt about.