@atLas: thanks for the clarification. I think we both end up in the same place, that the CX pass is not the tool for this OP's job. :-)

@CrazyInteg: I'm hoping we've dissuaded the OP from including SGN at all on this short trip--it just doesn't work so well and takes away from other opportunities. And adds costs and visa issues.

OP hasn't gotten back on whether PNH is a valid entry or exit gateway for his award ticket, but I'll jump the gun and make suggestions for a transportation scheme that works in his time frame and is economical:

Option A--PNH is a valid gateway (if KA on the HKG-PNH leg can be used):
--Fly into PNH. Depending on time of arrival, either get car to Siem Reap or spend night in PNH and bus/car next morning. (luxury bus $20 for 2 people, car $50 for 2 people)
--Siem Reap, 3 nights
--Overland to Bangkok (leave am, multiple ways to do this), transfer to airport, early evening flight to HKT on Air Asia (TG, PG, other options, should be doable for $30-50 pp for the BKK leg and $60 pp for the flight leg) Overnight in Phuket (or Krabi)
--Phuket/Krabi 3 more nights
--Air Asia morning flight back to BKK (another $60 pp). Options include staying in BK or if that's been done, head for Kanchanaburi, Erawan Ntl Park, etc. (total of 3 more nights) Fly out of BKK to YUL. Depending on methods of travel, all the intercity stuff could be accomplished for $150-200 per person. If really overweight on the baggage, excess luggage can be parked at BKK, I think it's 100 Baht per bag per day...maybe better than paying Air Asia excess weight and/or wrestling with it on overland segments.

Permutations of Option A esp if BK has been visited before, include flying from HKT to CNX on Air Asia, then return CNX-YUL or separate flight CNX-BKK then BKK-YUL award. Or, After Siem Reap-Bangkok, stay there or head west or north, then fly to Phuket/Krabi and fly HKT-YUL on award. This entire idea can also be done in reverse starting in Thailand, ending in Cambodia.

Option B--PNH not a valid gateway/can't use KA:

Get award in/out of BKK. Do a 2-loop trip in any order (I'd tend to put Cambodia first).
--Fly Air Asia BKK-PNH (prob about $100 each). Overnight in PNH.
--Morning bus or car to Siem Reap. 3 nights.
--Overland return to BKK as per above.
--Loop BKK-HKT/KBV-BKK (or CNX-BKK) as described above.

In addition to the award tickets, the Air Asia flights should be booked well in advance to get the best discounts, but all the land part can be done on the spot. Also, check out TG and PG, as sometimes they have specials esp on the Phuket route, that aren't much more than FD and the reliability/adherence to schedule is better.

Note on overland travel:
--Phnom Penh city-Siem Reap is 4.5-5 hours by bus with a pitstop partway. Road is good. A private car can accomplish the trip in 4 hours. PNH is about 20 minutes from central city if traffic isn't too bad. I'd tend to recommend a stay overnight here anyway, even a half-day or day to catch a couple of sights and look around.

--Siem Reap-Bangkok is now on a good road all the way. The old days of the legendarily pot-holed and washed-out Cambodia stretch border to Siem Reap is now over with. This trip can be done in abut 7 hours but allow an extra hour at border to deal with any immigration queues and to switch from Cambodian to Thai vehicle. I recommend overlanding Cambo => Thailand rather than other way around. Many experienced travelers prefer to just get a vehicle (bus or shared car) from Siem Reap to the border, then get into Aranyaprathet and change to government bus or train (slower) to Bangkok rather than the private minibuses.
www.canbypublications.com is an excellent resource of Cambodia travel.