If one forgot to shave (assuming a kosher electric shaver) and/or clip nails before midday on erev Pesach, can one shave oneself and clip nails after midday? What about other holidays like Succot and Shavout? Or does it apply only to Pesach?
Yes, after the fact one may shave and cut nails after midday Erev Pesach. The custom to refrain from certain melacha activity after chatzos only applies to Erev Pesach being that the korban Pesach was being brought at that time, it obtains the status of a semi yom tov itself.