Is it permissible to repent and begin fasting for expiation of accidental killing after drinking alcohol, or must one wait 40 days because prayers and fasting are not accepted during this period?

Expiation is obligatory in cases of negligence and dereliction, not in their absence. The non-acceptance of a drunkard's worship applies to one who has not repented, but a penitent person is not included in this threat. The meaning of non-acceptance is the absence of reward, not the invalidity of the worship. One must hasten to repent immediately and to offer the expiation if its obligation is established.