Divinity is a waste of 5 points. Heals will be sufficient so extra would teeter-totter between hinderance and comfort in regards to talents.
Conclusion is split on this. On one end you can talent for more damage = more holy damage = more threat built with just Crusader but to the point of certain mobs. But then your threat might suffer on other fights like Beasts and HM Ulduar if you do those. It can be marginal but it's up to speculation.
Divine Guardian is handy yet not officially needed. But being a tank, might as well have it in your utility in case of splash raid damage.
Healing is the source of life. Without it, you're left with death.