Is it sufficient to forgo seeking forgiveness from those one has previously slandered, and instead rely on giving charity on their behalf and supplicating for them during virtuous times, in lieu of seeking their absolution, which one fears could lead to greater harm?

We have previously clarified how to repent from backbiting, and that it is not necessary to seek forgiveness from the one you backbit if doing so would lead to a greater corruption or harm. As for mentioning the backbitten person positively in gatherings, this is also not necessary if it entails corruption or harm. What you have mentioned of deeds is sufficient.