Conductive paints are a gamble. You can do them with a brush instead of a spray, which is better, but... still a crapshoot. Many conductive paints require multiple coats to get any real conductivity.
i have MG CHEMICALS 841WB-150ML NICKEL CONDUCTIVE COATING bought from digi, its worth it sure better than nothing, but you need airbrush spray tool and compressor, tubing etc... using brush is a no no as conductive particles will just stick to the brush, not the enclosure to be coated. and EM protection is in term of dB, more layers, more dB attenuation, maybe 2 or 3 layer is ideal, more than that you wont get further noticable protection. if you tight arse, 1 layer can give you some protection already. ymmv.
It's a lot of work and likely some headaches
yes, painter's skills are really needed to be applied here, covering/masking from spray here and there etc... and conductive particles is special type, we need to keep stirring spray tank on each spray pass otherwise they will settle to the bottom very quickly and clog the spray tool.
If you look at the design of the N2795, there is a hard plastic part and a soft silicone rubberized part, where the MK1 bionic grabber is supposed to handle the probe. I'm going to guess that this part is shielded for sure.
most likely yes, my agilent 1141a has silver coat on inside of enclosure, its durable and scratch resistant type, unlike my water based coating type. ymmv.