at this stage we will be arriving on a Monday night

Malls close at 9:30 - 10:00 PM and re-open at 10:00 AM, so doubt you'll make it into one assuming you land at 8:00 PM.

You might be better off just staying at one of the budget hotels near the airport. (Note that some "airport" hotels are quite far away from the airport, with transit times of 30+ minutes.)

A 12/13 hour layover can quickly translate into 7/8 hours once you factor in Immigration (arriving), baggage claim, transport - return transport, checking-in, etc.

I recently waited 15 minutes in the Priority Immigration line (only two desks open), and another 10 minutes for my luggage and another 45 minutes in the official taxi queue, got stuck with a larger taxi (Innova), shaken down for 700 baht (I paid 500 after some veiled threats), and experienced speeds of 140 KPH on the highway.