Is it permissible to delay wiping over the socks after performing ablution upon exiting the restroom, while washing the hands out of fear of the doorknob being impure before wiping?

Ablution (wudu') is valid, and the act of wiping over the socks, even if it's not in the same location as washing the rest of the limbs, does not affect its validity.

If the socks are worn while in a state of purity (tahara), then it is permissible to wipe over them: for a resident, for one day and one night; and for a traveler, for three days and three nights.

The default state of the doorknob of a restroom is purity, until one is certain of its contamination with impurity. The questioner is warned against excessive scrupulousness (waswasah) regarding purity.