I have not flown Maswings this year, but based on my understanding and experience:

-no lounge (BKI now has a lounge, which can be accessed by DP/PP);
-no extra baggage (and on DCH6 they weight you and your carry on too for weight and balance purposes);
-probably can use the Jcl counter (if it exists at those small airports, BKI has it for sure) and the agent maybe more helpful to get you the seat you want;

But emm, is Mulu operated by DCH6 or the ATR? In case of the former, its essentially you just sit wherever (even though the BP has a specific seat indicated). If it's the ATR, I don't remember them charging anything for any seats, they are all the same, although I prefer to sit at the back whatever the aircraft is.