[understand that upon deplaning the will be taken to the Emirates Business Class lounge]
I don't know about being "taken", but if they aleady have the Boarding Pass, they can go through Transit Control to go air side. Not sure how they'd then later exit the airport. I'm sure it can be done but usually they don't make it convenient.

[How early do they need to be back to check-in]
There is a dedicated Business/First Class wing of the Terminal. I've never had any queue through Immigration. No need to arrive at the airport more than 2 hours prior.

I don't understand the idea of roaming around for 16 hours. There are a couple of nice, inexpensive hotels a 5 minute shuttle bus ride away. $100 for a hotel is nothing compared to tickets and the Maldives. I've done the exact same itinerary, even with getting many hours of sleep on the plane, I've still had to catch some sleep between flights.