About blizzard and d&d :
Knowing that difference will matter if you are playing humans a lot. If you are going to use your blizzard against farms you will need multiple mages because otherwise farms wont die it is very ineffective to use blizzard against farms as a human this is just not a viable strategy really.
One blizzard will leave farms alive where one d&d will kill them all that is the thing, often late game you will see dks come and rip apart patches of farms, this can completely cripple a late game economy 1 mage can simply not do that it is not an option.
If there is a difference to be seen between the two spells it is on farms that you will see it the most. Units will die fairly quickly to both d&d and blizzard but farms will not die under 1 blizzard.
Multicast blizzard does not own multicast d&d in my experience they both annihilate pretty much anything, but arguably hasted d&d owns both because you have close to the same effect and it is much cheaper to achieve.
Blizzard will be able to kill ogres a little faster yes but both spells will kill an ogre when multicast comes into play, both races can multicast it is not just a human thing.
Again not saying blizzard is < to d&d more that these two spells are pretty balanced as it is things do not have to be exactly equal to be balanced.
About Coil :
Saying a full life dk is the same as a 1hp dk is just not true, not every unit in the game will one shot a full life dk as a matter of fact dks can even take several hits from non lusted ogres before they die coil just allow you kill and sometime even survive a situation that d&d wouldn't. On a water map this is the difference between loosing your one dk defending a spot and keeping it fresh for the next attack that will happen maybe long after when the dk will have recovered all its mana back.
When facing another dk if one casts d&d and the other coil the coil guy will survive d&d and kill the enemy dk because of that life back otherwise it would be a draw.
All the tier one players know how to use and abuse it, coil is a very effective and useful spell and it is used a lot before and after d&d is researched, and it not as unusual as you would think that coil changes the day, especially if you look at water maps where the late game really involves casters more.
Now if you look at fireball as a comparison it is barely ever used before blizzard is researched unless its a life or death situation and certainly not after unless it is a troll the spell is barely ever used at all it is just a bad spell.
As a matter of fact it would be like if dks had Skeletons as their first spell instead of coil and had to pay/research coil in place of skeletons except even skeletons would be arguably better and more used than fireball is