Based on your limited profile, I assume that you are based in the U.S. or Canada. The Bangkok Airways website has U.S. & Canada toll free numbers that route one to a call center in India. They book the Discovery Pass. Since Discovery Pass is capacity controlled, be prepared to make decisions re: your routing while on the phone or at least book segments to hold the ticket. They will make changes for free until you send your certified check in. Be sure to book air before hotels because of the capacity controls.

Last November, I flew BKK-PNH ($80), PNH-REP ($50), REP-BKK ($80), BKK-LPQ ($80), LPQ-BKK ($80), taxes extra. Each international BKK departure incurs baht 500 ($12.50) tax payable at airport.

Unless something has changed, Siem Riep airlines (or Bangkok Air) fly the Phnom Penh - Siem Riep routing. Last year it was 3-4 times per day. Aircraft is an ATR-72. Sit in back rows to get off first.

Your plan is pretty ambitious because of travel times, connections and no thru routings available. BTW: Chiang Mai really requires only a couple of days. Routing thru BKK can be a pain if a plane is late. Connections may require that you retrieve your baggage and check in for the next leg. The call center will tell you what they consider legal connections. They are considerably longer than one would expect for an on-line connection.

USM-BKK and BKK-USM operate many flights each day. CNX is served not only by Bangkok Air, but also AirAsia, Nok Air and Orient Thai, as well as TG.
USM-CNX requires a change in BKK and it is actually cheaper to fly one of the LCCs between BKK-CNX to insure that you do not exceed the maximum six (6) segments allowed on the discovery pass. One can presently buy a 1700 baht +++ fare USM-BKK on Bangkok Air on the web-site and save a segment or two. Beware, only available on 2 very inconvenient flight per day. You may have to overnight in BKK to make connections unless particularly on the USM-BKK-CNX route, or spend most of the day to get to CNX.

You may want to PM me about LCCs and Discovery Pass. I am presently routing USM-BKK-CNX-BKK in October. To get to USM, we are flying Nok Air business class ($46) BKK-HKT and driving to Samui from Phuket after, of course, a six day stay at the J.W. Marriott, Phuket.

Last year I was able to visit BKK, HKT, KUL, PNH, REP, LPQ, and Macau/Hong Kong in 3 weeks. It required a number of overnights in BKK, 6 Air Asia Flights, a round trip hydrofoil trip and a full Discovery Pass (6 segments). I spent 2 nights in PNH, 2 1/2 days in REP, 2 1/2 days in LPQ and almost a week in Macau and Hong Kong. Most people might feel overwhelmed by such an agressive itin., so be cautious about what you plan and what you are able to execute.

Please do a search in Trip Reports for "Luang Prabang" (I posted a partial review of the above trip under the heading of "Greetings from Luang Prabang") and in Asia for more information.