ride red,
let me try to help you. i go through bkk several times a year. the fastrack thing is truly a circus, it seems to change everytime i go. so be ready to have option B, C, etc... first of all, you are arriving towards the end of a busy period, arrivals 10pm to 12pm. even sunday can be crowded but probably less btwn may and nov. are you checking bags ? if so, my guess is your bags won't be on the carousel for at least 30 minutes. if you have to go through the normal immigration channels (assume no special line for C boarding pass), you won't wait more than 30 minutes. my guess is no more than 15. i don't recall immigration lines being the hold up in bkk when you check bags, it is usually the wait for the bags that kills you. yes, the lines can seem long but then you just wait for your baggage.
i almost always arrive at the C concourse using TG. the immigration hall near the C concourse has had a lane for fastrack/diplomatic in the past. due to circus, you have no idea if it will be there tomorrow (or even now, etc...). so take a look at the first immigration hall you come to. you can (i) try the fastrack lane with the boarding pass, if such lane exists (ii) get in the regular line if it doesn't look very long.
if that doesn't workout (no fastrack, overcrowded), walk towards the E concourse and you'll pass immigration hall #2, which is the real diplomatic/premium passenger area. i don't think they'll take your boarding pass but you could try. i went through there once with TG but i think TG can't go there anymore (but again, who knows what the deal will be tomorrow or next month......).
if you are shut out at the first hall and the second hall, then you end up at the last hall near concourse E. if you are lucky, the lines there will not be as full as they were at the first one. they may or may not have a fastrack.
i've had pretty good luck bypassing the hall near C concourse for the hall at E concourse (when traveling economy, no premium lane access). i think one reason is the people who bus in from the planes get directed to the C concourse. they get dropped off and go up an escalator, then keep walking straight to the immigration hall in front of them. what they don't know is there is another hall behind them and they can make a u-turn at the top of the escalator to get to it. so they bus in 400 people and all those passengers go to same immigration hall. very few go up the escalator and take a uturn at the top and go to E concourse immigration.
when you land and get inside the terminal, verify which concourse you are in. if you are in C, great. if you are in E,F, or G, then you do your first check in the immigration hall next to E, and do the above in reverse.