Shield of Faith - as a Reaction does it last 1 minute?

First:
Shield of Faith cast normally has a duration of 1 minute. All attackers must roll twice and take the worst roll. (caused by Disadvantage)

That's super-powerful right?, and just as powerful at level 1 as it would be at potential level 30.

Question 1: It says you can cast the spell as a Reaction. Does it then also have the same duration of 1 minute? As written it seems so. It seems like you'd never have to spend an action to cast the spell, it's always better to cast it as a reaction.

Question 2: As you level up you get the ability to cast more and more level 1 spells. The other level 1 spells have limits and actually decrease in usefullness as you level up: Spritual Hammer only ever does 1d8, Searing Light only ever does 4d6, Divine Favor is +1 to attack. Assuming monsters HP and defenses start to scale at higher levels. But Shield of Faith is just as powerful at level 1 as it is at level 30, plus you get to cast it as a Reaction so you don't waste a precious Action. So it would start to trump all your other level 1 spells. Is that intended? Are Reactions going to get more precious as you level up?
Reactions, by definition, require you to forego your action on your turn in order to prepare. "Completing the magic incantation and prayer, Gersi the Cleric awaits to finish the spell Shield of Faith and cast it on the first ally to be struck in combat, as a reaction."
Reactions, by definition, require you to forego your action on your turn in order to prepare. "Completing the magic incantation and prayer, Gersi the Cleric awaits to finish the spell Shield of Faith and cast it on the first ally to be struck in combat, as a reaction."



I don't think that is the case. The rules say "The most common way to take a Reaction is to Ready an Action" but a Reaction is just like the good ole' 4E Interrupt. It says some spells can be cast as Reactions, it says nothing about that using up Actions. Re-read the Reactions section p9 and the Casting Time section p23.

It's important because the Kobold Dragonshield is Elite because it has a Reaction. If that Reaction also took it always readying, why would it be Elite?
I could be wrong, but spells/attacks that can be used as reactions don't need to be Readied. 
"At a certain point, one simply has to accept that some folks will see what they want to see..." Dragon 387
Based on their example of using Feather Fall as a reaction when you fall into a pit, I'd say reactive spells definitely don't need to be readied.

Readying an action costs an action to do and then resets your initiative to be immediately before whatever it is that triggers your reaction.

Since reactive spells do not require readying an action, they are basically a "free" action on something else's turn; they don't cost you an action nor do they change your initiative order.  However, the rules do say that you can only take one reaction until the start of your next turn.