Well-nourished usually refers to the person being of normal or above average weight, rather than skinny or anorexic.
No, the women I've known closely are not model types who spend a lot of their time preening themselves. They've all been practically inclined with shortish nails. My mother had koilonychia for several years, which made her nails very brittle until her doctor diagnosed pernicious anaemia and prescribed iron dextran and cyanocobalamin by injection. But even mild anaemia can affect nail strength, and this may have been present but not picked up on by Sheila's doctor.
All we have is a very blurry image of her long polished nails, and the pathologist's statement that they were undamaged, unless there are other pictures you know of. If photos of her clean feet taken during post-mortem have turned up, other clearer shots of her hands and nails could exist which only the police, pathologist and lawyers have seen.
I've no doubt that Sheila might have been able to load the magazine, possibly without damaging her nails... if, but only if she'd been shown how to and was very familiar with the procedure, but according to her husband she hated guns and never used them. Whether she could have done this under the extreme pressure and speed required during an assassination without damaging any of them is unlikely in my opinion.
It was Sandra not Caroline Elston who also said that Sheila was lacking in confidence, gullible and insecure so she always seeking reassurance.