I think it would be nice if the icon names wouldn’t just say what the icon depicts but would say more what the intention of the icon is. This would make it much easier to replace it by a different icon in another icon theme.
For example, what’s bell-slash vs. bell-empty and when would I use them as opposed to the bell icon? An icon like not-following or custom-notifications would be much easier to use and replace in my opinion and would avoid that we use the same icon in different contexts.
Regarding the Silk mappings, I think we should either go forward with making Font Awesome the fallback icon theme, or we should create a custom Silk-style icon. But again, the problem here is that the icon’s name doesn’t convey in any way how it is used and thus makes it impossible to replace easily. If this was following-custom, we could, e.g., combine a bell with either a wrench or a pencil to denote that the notifications were customized.