Possibly.
How about long nails loading bullets... And fighting Neville and no broken nails...
One thing that this case teaches you more than any other is that you can never assume that what 'everybody knows' is actually true.
Everybody says she had 'long' to 'very long nails', but the photographs of her body do not show this. Look for yourself.
In any case, why must long nails be an obstacle to loading bullet cartridges, if she wanted to load cartridges? I can see why they may make it more difficult, but it doesn't make it impossible.
Why would she have broken nails if fighting Nevill? Women with long nails (assuming Sheila did have long nails) can fight without breaking their nails.
It's also never considered how Sheila having long nails (if she did) would be an additional advantage for her over Nevill. Other advantages would be the fact she is a woman and his daughter, making him more reluctant to tackle her, and the fact she was much smaller and lighter than Nevill and probably fleeter of foot than he.
But why assume she had to fight with Nevill anyway?
On the matter of grease, I assume you are referring to the beeswax, which is used on Eley ammunition as bullet lubricant.
The defence case theory was that Sheila washed herself. This was supported by an eminent pathologist who gave the opinion that a murder-suicider may ritually wash herself. We have the hand swab tests on the Forum and David has commented on this and said that the results do not preclude Sheila having handled and used ammunition and a rifle, but in practical terms, I am not convinced she would have got much beeswax on her hands or that a swab test would detect this to any great extent.
She was picking up the cartridges and placing them immediately in a magazine. Wouldn't the wax residue stick to her fingers, rather than palms? Were the fingers swabbed and tested? Isn't it likely that she would probably wipe the beeswax off her fingers of her own accord, perhaps still leaving some traces on her fingers, but possibly not enough to be detected?