Our flight will be arriving at 6:00 pm.

We figure 7:00pm to 7:30pm to do all the required airport and auto rental stuff and get to Den Laman.

The question is - can we make it to a grocery store or is that wishful thinking?

David,
The larger grocery stores close at or before 7:00 pm on Saturday, according to the info I have.
There are a couple of smaller stores that stay open later. Wing Cheung Mini Market is open until 9 pm, and Exito N.V., The Bakery stays open until 8 pm.

Den Laman may have a grocery pack that they have in the condo when you arrive. It will be a basic package just to get you started. Email them and ask what they offer and what the cost would be.
Several of the resorts have this service for late arrivals.

Sand Dollar Grocery is right by there too, don't know what time they close though at the moment.