1. You CAN check your bag directly to KUL. A 16-hour cxn is not unusual for BKK.
2. You DON'T need to clear customs and immigration, NOR claim your bag, if you decide to remain airside. BKK is open 24H. Even during the wee hours you would be surprised at how much activity is going on inside the terminal. Some stores and restaurants are open (IIRC).
3. You can use Royal First, Royal Silk, and Royal Orchid lounges, but not Royal Orchid SPA. There is a gap of about 3 hours between 2-5am when the lounges are closed. However, there is a 24H fee-based lounge (IIRC), or two.
4. I'm not sure if I would go into BKK on a 16 hour cxn, but that's just me. There's too much to do in town, and I wouldn't want to return to the airport!