Why does shirk have a mana cost? Why does "shirk 1" prevent "2" damage? And what does the trigger trigger on? Preventing 1 damage? Preventing 1 or more damage? Arriving at the meximum amount you could prevent in a turn?

Is the additional cost to mill yourself necessary?

Also the missing comma before "you may" makes reading this weird.

Self mill wasn't necessary. I can be a bit paranoid and tend to unnecessarily nerf cards.

I believe absorb was considered too strong to use. The mana cost to turn on shirk was my answer to balance absorb. The "2" in parenthesis was just sloppy copy pasting from another shirt card, since I didn't add the mechanic to the mechanics page.

A comma would look weird before "you may" in my opinion. Looking at the text again however, I see I missed the comma after "do."

EDIT: I think at one point I intended multiple uses of shirk to stack. Since you'd be preventing damage and tapping down a creature, I wanted some kind of drawback on the creature.

So does Shirk actually read: "Shirk 1 (: Until end of turn, ... this turn.)" ? Confusing as heck.

You could just go with ": ~ gains absorb 2 UEOT".

I think it comes back to the "Why doesn't this just boost toughness instead?"

I also have to point out that absorb is a rather bad mechanic. You don't have to scour long in blogatog to find many negative notions about it:
http://markrosewater.tumblr.com/search/absorb

Whatever Pestilence combos you have in mind, I highly doubt they're worth all the trouble it would take to salvage absorb.

Having gone through the Blogatog search the main points MaRo brings up about absorb are developmental ones: absorb is too good and slows down the game. Having a cost should mitigate those developmental problems since the player must have to decide whether they want to shirk or cast a spell. Additionally shirk only prevents damage on the first time the creature would take damage that turn as opposed to absorb which reduces the damage on each instance of damage.

The reminder text wording may be off or not as clear as it could be; I'm often amiss with that.