FSA issues when employee goes on unpaid leave

[Silver70]

My mind is going blank on this, as i thought i knew this.

1. When an employee has an FSA and is on Leave of absence without pay, are they required to make their FSA contributions to keep "current"? Can they if they choose?

2. When the employee returns from unpaid leave, is the employer permitted to "catch-up" the amounts the employee missed?

3. If an employee is going on a leave at the beginning of the month, is the employer permitted to double up the contributions on the employee's pay in order to get the current month's complete deduction?

Thank you,