Spurning Spurn ... and Bringing in Detain?

I think I've hit a "Kill Your Darlings" space. As much as I want to make "spurn" work as a mechanic, it just really isn't fitting into this set. Flavorwise, having it trigger from spells and activated abilities seems very much not in the spirit of a world that's worried about being overwhelmed by beasts and plagues. Mechanically, it doesn't create a win condition or really set one up because the effects are so varied and so reactive. White in particular is struggling for a play archetype given that it doesn't have access to either mutate or reave.

So "spurn" is out. There may still be spaces in the set where I can have white and blue punish opponents for lobbing spells at them. That fits well into what they're about in this set. I'm thinking of bringing detain in to replace spurn, the Azorius mechanic from Return to Ravnica. In the big Magic Design Search 3 essay test, we were asked to pick a mechanic to make evergreen. I chose detain, because it mechanically clarifies and clearly represents a host of existing stun mechanics that blue and white already use to keep opponents from using their creatures. So could I make a detain a mechanic blue and white use to hold opponents' creatures back to prepare their own armies?

I have decided I do want to make the burgeoning enchantments work. They actually better represent the growth concepts of this world than spurn did. I'm still not entirely sure about balance, power, rarity, and what to do when the options run out. I don't want them to just pop like a limp balloon. So I'm thinking at the very least, they cantrip once they're done.

Also an interesting bit of tension once you bring detain into this set. When you target a mutating creature with detain, it's going to get a +1/+1 counter. So you've successfully held the beast at bay, but it's feeding off your magic.

I decided to go with a "may" version on this one, and I'm wondering if it's going to cause memory issues. A player can decide not to use the first effect when his next upkeep comes around. So it will remain at that first effect for his or her next upkeep. Is that something a player would obviously realize?

Edit: I just realized that if it's a "may" effect, they can just leave it in play and never select the last step, which seems weird. I think the "may" has to be on the steps themselves. In which case I'm wondering if a player thinks they've chosen a step even if they decide not to detain any creatures. Complications!

Edit II: Since the steps require targeting an opponent's creatures, I think the "may" is not necessary. You don't have to worry about it backfiring on you (which would not be in the flavor for this one). It would just fizzle.

But fizzling creates a new gameplay concern. If an effect fizzles, does that still count as "chosen"? If there's no valid target for the first effect, what happens? Here's where things get messy. Normally you cannot cast a spell or use an ability when there's not a valid target.

So under the rules, can you even "choose" the first effect if your opponent has no creatures? So perhaps the first two items on the list require a "may."