Does sin befall me for burying the deceased without offering the funeral prayer over him, knowing their refusal to perform it, and my burying him alone despite my young age, and my declaring disavowal of their action?

Whoever buried the deceased without the funeral prayer has erred, as it is a communal obligation (fard kifaya). They must repent and seek forgiveness. They are obligated to pray over the deceased at the grave. For the Shafi'is, it is stipulated that the one praying over the deceased at the grave must have been among those obligated to perform the prayer at the time of burial. As for the Malikis, they permit praying over the grave as long as the period has not been too long. If it is impossible to reach the grave or to find someone to pray over it, then there is nothing upon them except repentance and seeking forgiveness.