i would like to know if my new pes status will require me to acquire a new vocation, or if i could still continue as a driver.
I doubt the SAF will re-train you again, unless the system's undergone a complete policy makeover since my time. It involves just too much coordination/logistics to underwrite for such infrequent casualties. My guess is you'll get redeployed elsewhere within the same unit, subject to vacancy (CQ, mess boy, et al). That way, you're nobody else's "problem" since you don't have to be posted out.
That's what happened to my Arty mate, also a M113 driver. He ended up following the rations truck - not as switched-off as you think - all the way through reservist service. [BTW In reservist, a downgraded soldier won't even get redeployed, internally or kicked out, unless he's really a hard-up case; the most they'll do is excuse you from IPPT & formal parades & stuff. That's my experience, anyway.]
"can my csm force me to continue as a combat driver regardless of my pes status? or is he prohibited from doing so according to the policies of the saf."
No, it's outside of his control, since the downgrade is done by MMI. Talk to your BC or PC, once your medical center has validated your new reclassification.
"why am i given a temporary downgrade only?"
No idea; flat feet are evaluated on a case-by-case basis, I guess. It could be 6 or 9 months next time round. Maybe your MO can explain the diagnosis, once he's able to read your new medical docket.