Does missing congregational prayer once prevent one from attaining Laylat al-Qadr, or is its attainment through something else, especially given the inability to perform i'tikaf?

There is no doubt that backbiting is one of the gravest major sins. Its perpetrator must seek forgiveness from the person they backbit. If they fear harm, they should repent to Allah, mention the good qualities of the person they backbit in the places where they backbit them, and supplicate for them. As for missing congregational prayers, this does not prevent a Muslim from attaining Laylat al-Qadr if they strive to seek it through worship in Ramadan (definition), especially if missing the prayer was due to an excuse. Nevertheless, one should be diligent in observing congregational prayers and attaining the opening takbir due to their immense virtue.