1) No, holdings do not give you knowledge about military units, they do however give you the option to spy on the provinces, looking for military units. Other than that, you need scouts.
2) It depends on the domain in question. A Haelynic or a Cuiraecenite temple most likely have deals with the realms wherein they are the major temples. Other than that military units are usually allowed for defensive purposes, garrisoning your court etc. Anything else needs at least a dispatch asking for permission (if you have good relations) or diplomacy if you haven't.