Sure you would!Technically it would be 'General Earl of X' and less formally just 'Lord X'. You certainly wouldn't create hybrid titles such as 'Lord General', though it sounds cool so perhaps in a fantasy world they might do that.
Depends who they are. Prince Harry was Captain Wales, for example.From extensive reading of novels and diaries set in the Peninsular Campaign, I'm pretty sure you'd address them by their title, not their military rank. Even in a military situation.
Generals are, in general, the general head of the army. It is a shortening of captain general, which was formed along the same lines as attorney general and Estates General and other post-positive adjectives. Generals are therefore generic and genetic, ruling over a genus of soldiers