An alternative approach in case you don't have Joker skill (since it's a very situational skill, and one might not necessarily have room for it in their build) is as follows: (assuming you have the smooth talker skill)
At this point, you have entered the bathroom and killed the bathroom guard in a panic. Take a second to breathe. You now have 4 wandering guards, lobby guard, and cameras, with only 3 pagers and a dominate. We have established that you don't have Joker, so you can only take care of 4 guards.
The plan is as standard: Kill 3 wanderers, dominate 1 (however you want to accomplish this), the only caveat is that you have to ignore the lobby guard, break the cameras, and throw the paintings to the roof (you do have the transporter ace, right?), where your other crew members are waiting to catch them (or, if you are doing this solo, you can break the skylights and throw the paintings to the roof. I am told that the lobby guard is deaf). Do note that the lobby guard tends to spot people on the roof extremely quickly, so stay far away from that skylight. Hug the walls and you should be fine. Have the crew van the paintings, and sneak out through the bathroom.
As I said, this is a very plan-b approach, and it leaves one guard in play, but it's definitely doable. It just requires a little more care.