I don’t know if this stands true for Holy Paladins before level 60, but they’re effectively 1:1 in terms of value, just because of the talent Illumination.
+Healing is basically spell power, or Intellect in retail. In Classic, however, Intellect doesn’t increase your spell power, and instead increases your maximum mana and your crit chance with spells.
Every spell has a spell power coefficient, so a percentage of your spell power is applied towards the spell’s healing. It’s not always 100%, so +100 healing doesn’t mean you always heal 100 more. It depends on the speed of the spell.
Every spell has a base cost and heal. Lower ranks are less efficient to start with, but because of the spell power coefficient being the same, they start to become more efficient in terms of healing per mana spent (HPM) as you gain +Healing.
Exactly.