My nephews one golden retriever eats those cut clumps of grass and mulch now she has the other one eating them too. They have tried to get them to stop and they will as long as they are looking but once they turn their back there they go again. LOL I guess that means that I am no help on either topic
I have had mine help me dig holes when I plant a tree or a bush. They love it. I just have to micro manage on the rest of the time so no one digs holes in yard. Usually if I catch them I tell them to leave it. I would think if you are vigilant enough you could train them to just dig in a doggy sandbox but you would have to work on it.
Someone else on here has a sandbox and has taught their Boxer/s to dig just in that, so i know it can be done, just would take work...I have 3 grass eaters here, so I'm no help with that and Angel just loves to roll around in the lawn as well