not from the faculty but a current student there. i would say there isn't a best time to submit an application. just make sure the application reaches their hands before the dateline.
i'm not sure how many percent of international students were accepted out of those who applied, but i don't think its that hard to be accepted by IMU. As long as you meet the minimum requirements, you should be called for the interview. And the interview is just basic questions. i was asked to introduce myself, talk a bit about my family, asked why i wanted to do medicine as opposed to the other life sciences courses, then was asked a scenario based question which is very easy (you're a doctor. you just saved this patient's life from the brink of death. he rewards you with a full expense paid trip overseas. would you accept?).
don't be too hung up on your summative results. concentrate on the eos (and don't waste your 4 weeks!!)

skip the bs stuff if you have no time (i remember only a few oba questions coming out from bs).
but as for physiology, pathology and pharmaco.... each one is equally as important.
as for osce... i just can tell you to practise practise practise. remember GIEP (greeting, introduction, explanation, permission!! each earns you marks), washing your hands and warming your hands before touching the patient.
when washing your hand... make sure you don't put too much soap. takes time to wash it all off. 
all the stations are pretty straight forward (much easier than the mock i remembered. they'll tell you to palpate+auscultate, or palpate and percuss). so follow them exactly. no extra marks will be given if you do anything else.
for meq... try not to panic and think carefully (but not too long) before writing the answer down. i find that many (myself included) start scribbling down the answer only to realize what we were writing is wrong. esp pathophysio qws
ospe.. as always is horrible. as opposed to meq.. i recommend to just scribble down whatever that comes to mind during an ospe paper. you can always review the qws you were unsure about in the rest stations.

can't remember anything else.

They'll call you up and let you know the date/time.
By the way, why August?? I thought Feb is the next intake?

Using IMU-PMS, lets say if you apply for Melb/ANU, you need to go through Med Sc then procede to the 4 year program at Aus. If he gets a direct entry, then can save the $$ spend in M'sia.