I haven't seen Redwood or McPlumb post in a few days now, so I'm not sure if they're still here or not.
So far as I know, there is no "head" of water needed upstream of the p-trap. Typically, on most sinks, the p-trap is located less than a foot below the drain.
If you back filled with sand, then any concrete you put over the sand would probably break loose. If it were me, I'd be inclined to make up a weak "concrete" by mixing something like 10 parts perlite with one part white cement. When that mixture cures, it's strong and hard enough to stand on, but you can easily remove it with cold chisel. You'll need to clean the sides of your exposed concrete and use some sort of bonding agent (called "concrete bonding agent") to get the new slurry to stick to the old concrete.
Or, partially backfill with sand, and then cap with a weak cementatious mix.
If your drain pipe is only two feet long, I wouldn't worry about getting a 1/4 inch per foot slope to it. I'm thinking that the ABS fittings you're using might not even allow that much slope in the pipe over two feet.
But, I'm not a plumber. Hopefully one of the plumbers in here will chime in.