Lower armor means you gain more from protection since protection reduces whatever is left after armor reduction. So casters gain relatively more and thus rogues can't kill casters as fast as they should be able to.
No. Now this depends on how HC does armor, but either way it's the same. If HC uses vanilla MC mechanics, protection enchantment provides a % damage reduction. In the case of a wizard with full leather and max protection which is 40-80% (Number is random, because vanilla is derp). Say the wizard is hit for 100 damage:
In Vanilla:
Armor reduction: 32%
Protection reduction: 40% to 80%
((100-32%)-40 to 80%)
68 - (40% to 80%) = 40.8 to 13.6 damage
Total Damage Reduced: 59.2% to 86.4%
Now if we where to do the same thing with say full iron:
Armor reduction: 60%
Protection reduction: 40% to 80%
(100-60%)-40% to 80%)
40 - (40% to 80%) = 24 to 8
Total Damage Reduction: 76% to 92%
If for example, protection armor gave a flat damage reduction in HC (I believe it may, but I can't find the exact system recorded anywhere):
(For this case we'll say that each lvl of protection reduces damage by 1 point.)
Full leather with of Protection 4:
((100-32%)-16)
68-16=52
Damage Reduced: 48
Full Iron with Protection 4:
((100-60%)-16)
40-16=24
Damage Reduced: 76
So yes, having lower armor and protection is more EFFICIENT, but it does not make them tankier.