SeamasterLux : do not have any hope for promo awards from CDG (or AMS) to SE Asia and to REP in january or february. This is the high season there and all these flights are packed. You can consider yourself to be lucky to have already secured what you have secured in term of award tickets.
Regarding the BKK-REP sector on PG, you are correct : it is very expensive. Until recently, PG had a monopoly on this route and charged a premium. Since a few months, Cambodia Angkor Air (K5) has broken this monopoly and operates 1 or 2 daily flights (can't remember). You may try (you can book on-line and they are reliable) but it may complicate your trip to KBV.

As a whole, I understand you feel going more than needed through BKK, but I believe this is almost unavoidable with your itinerary. BKK is the most convenient in your case and all those flights are very short (BKK-REP is 35-40 min flight time / 1hr block to block) and BKK-KBV is just slightly longer.