Diablo – not getting the full health regeneration bonus from the follower

I'd bet on the templar, because all of the four ability choices provide tank-like abilities at will. His taunt ability will keep enemies off you for 5 seconds.

Then, as soon as they hit the templar, his Intimidate skill will slow them down by 50%, making it easy for you to get out of range in case they target you again.

His Charge ability will enable him to intercept enemies that attack you, and stun them for 3 seconds in an AOE radius of 15 yards, giving you time to get out of range.

And last but not least, his Guardian ability will make him attack enemies for 300% weapon damage and stun them for 5 seconds if they attack you and you happen to be on low health. As a bonus, he heals you when he uses the ability, and also gives your character life regeneration for a limited amount of time.

Yes. Arcane Torrent does arcane damage, so the 2nd missile and every one thereafter will benefit from the damage bonus.