1) When Filipino MUST retreat after a combat round, do they suffer a Parting shot ?
yes. The only instance when a retreat does not trigger a parting shot is retreat before combat.
2) And can the defender decide to retreat even if he does not have to do so ? In this case I understand he suffers a Parting shot.
Yes. See 10.6.5, 4th dot
(David, I sent yo a mail bw )