yekke: Bimchilas K’vodcha, you are confusing several issues here. Biting nails on Shabbos is a Misasek because you are not at all paying attention to what you are doing. That is called a Misasek. We can get in to more detailed definitions (maybe we say Misasek by the Prat of an action, for example by Niskavein Lachtoch Es HaTalush), but the basic definition is if you have no Shaychus to the action being done.
Also, I’m not sure what your point by Niskavein L’hatiach is. That’s still a Misaseik, it’s just Chayav because SheKein Neheneh.