Diamond Store is easily doable with 2 people.
It's much easier if you're willing to kill most of the civilians on the outside. It hurts in the beginning, but after Rank 30 you can waste civilians and not even flinch at the clean up costs. (Fun fact: Pro Overkill Rats will take a Lvl 0 player to Lvl 31! in one run.)
Anyways, dead civilians or not, one guy picks the lock to the security room in the alley while the other covers them. Then take the guard in the security room. One player operates the camera while the other clears the alley ways of civilians (who will walk through the alleys), and opens the back door on the stairwell. Working together, you clear out the wandering guards from the top floor, and get the manager with her key. Once you have that, and you're clear of guards, deactivate the glass alarms. Then one player rushes the store while the other gets civilians out on the street, and the guy next door in the auto shop. And then you just clean house.
Things get a little dicey when clearing guards out of the top of the store, and getting the manager. But eyes on the cameras, coordination and patience, it's very doable. Just don't hang out in the 2nd level, go in, kill something, and move the body all the way out to the security office. Then wait out of sight of the door until you're ready to strike again. Again, killing the civvies in the back lot area also helps, I've seen them trigger on things as minor as LOS and silenced SMGs.
Things that are a must for all people IMO:
-Fast Hands Ace
-Transporter Ace
-Endurance Basic
-Smooth Talker Basic
Optional but handy things for people to have:
-Cleaner Ace
-Cable Guy Ace
-Dominator Ace
Everyone should have at least 1 silenced weapon available.
If everyone is packing all those skills, you can kill/phone in or dominate your way to having complete control of the map, and enough cable ties for most if not all of the hostages. Bags get moved fast, people don't get left behind everyone can do the critical parts of the mission if needed (kill someone quietly, hide bodies, answering pagers dominate guards you can't afford to kill and answer the pager on.)
Overkill is just straight nasty now. But Very Hard will be very doable once you've leveled up, fleshed out your perks, bought the better guns and modded them to the max.