I doubt it'll work that way. Usually, employees have to cover for each others' absence and they rotate the days they are off with other employees, so it's only a 3-day weekend when your day falls on a Monday or Friday. Somebody else gets the three-day weekend when you have to take off Tuesday. Otherwise, the whole office shuts down when everybody takes off on the same day. Some companies prefer to do that.
It depends. Count on the government to do this in a way that will maximize disruption and confusion. And especially anger at Republicans for causing it.
Actually it will. OPM (Office of Personnel Management) has decreed that all impacted Federal employees will transition to a 8 hour a day schedule and take every Friday as Leave Without Pay. No local discretion. You cannot even work 5 & 3 in the same pay period, even if it would be of benefit to the government. OPM does not trust the lower escholons of the government to do it right.