Weight capacity of drywall hangers isn't good. You really need to drive your anchors into the studs. I've had much better luck with magnetic stud finders used at the baseboard (finding the nails hammered into the studs) than with any fancy electronic gadget.

I use a skinny finishing nail to find studs. Just keep hammering in nail until you locate the studs. Stud finders have never worked for me, electronic or the magnet ones.

I have a Studpop magnetic stud detector. Works great.

https://www.thegrommet.com/products/studpop-magnetic-stud-finder?gclid=EAIaIQobChMIi4LSstjA6AIViJ6fCh1CxgZYEAQYDCABEgJVrvD_BwE#Color=Red

In terms of locating the pipes, you should be able to stand in front of the water heater and see where the pipes are. But I would be surprised if there were any coming out the side.

I'm thinking the only studs on that left wall will be at the very front and very back.

I use small, rare earth magnets as a stud finder. I run them across the wall and they find the nails in the studs.

I have successfully used drywall anchors with shelving on multiple closet projects in my house.