Yup. Close kitty friends groom each other. Very cute, but not very helpful for stitches.
Sleeping with the cones won't be a problem... Have you SEEN some of the ways cats will contort themselves to sleep? Lol. As far as eating and drinking, if they really seem to be having a problem you can take the cone off a few times a day, supervise them for half an hour or so while they have access to food and water, then put it back on. Obviously, more often just for water. Honestly though, none of the animals I have ever cared for while in the cone of shame have had issues eating or drinking. (I used to rescue and rehab "un adoptable" animals, so I have cared for several who were recovering from injuries.) They will usually only lick at it during the initial phase, when he sensation of the stitches and beginning of healing feels weird. Once they are used to the sensation, they will usually leave it alone.