Dumb question, I know, but I haven't run a Cleric once during my entire time playing Pathfinder nor ever played alongside one. The answer, I assume, will also apply to Spell Penetration feats and Crafting feats as well.
Yes they can.
Spells are spells, except when they're Spell Like Abilities. In other words, yep, you can take it.
Yes, and it is at least middlin common for clerics or oracles to take either spell focus necromancy or enchantment. (Sometimes conjuration if they are going for augment summoning but that seems rarer.)