So after reviewing various posts and opinions as well as suggestions for changing the rather unique ability damage, that is somehow drain enough to kill you (?) that a shadow does I've seen there are a few options.
1) Get rid of them along with the various instant death innerds liquifying monsters and impossible swarms (still working on that post folks! Actually I'm putting off the final version till I meet up with one next, which should be shortly from what I'm seeing in my crystal ball) that either died with the last few adventure paths of 3.5's attempt to make the game interesting and different, which only succeeded in frustrating people-bad business model!
2) Keep them, but take a gander at similar monsters, which is the mantra for monster creation, is it not? Let us suppose that shadows did not exist and some ambitious DM out there in the psychic hinterlands decided to whip some up. Could you get something doing all that at a CR3? Compared this to a wraith whose ability drain gives a fort save based on charisma AND has sunlight powerlessness. Keep in mind this level drain has a "counter" of sorts with a spell somewhat close (two levels for a cleric) to prevent this whereas for a 3rd level party, or a second level against one shadow the Adventure Paths can, any prevention is impossible to reach...or even a little mitigation.
3) Along the lines of mitigation, a simple spell, accessible at lower levels, that gives a sort of necromantic buffer like False Life against ability drain of a few points, say d4 + 1 every two levels lasting for one hour per level would go a long way to achieving this. How has this not been whipped up before? Hmm...I just thought of that on the fly and I really like it! Where are my research tools?
4) But back to the shadow-there was a post that an early version did 2 pts of str DAMAGE and then d4 cold. If you throw in a fort save, DC charisma based as with the wraith and they are looking a bit more reasonable still. Keep in mind on top of this they are essentially IMMUNE TO NON MAGIC WEAPONS! Wow! I wish I had that one going for me...How many characters are toting around magic weapons at 3rd level especially as el cheapo as a lot of adventure paths seem to be? And then on top of this they take half damage...ouch. So why is it the whole world hasn't become one big land of shadows by now?
5) Get rid of the whole spawning thing, essentially getting rid of the drain..oops I mean damage...I mean drain...what do I mean? Well I see that drain signifies that the creature draining somehow gets benefit sometimes but in this case, well. So get rid of the spawning, make it so it is just damage and the worst that will happen is you are too weak to stand up in your big tin can of armoring, or your component pouch weighs you down so you can't move, and the shadow would essentially cold damage you to death. Shadows would be created along the lines of how the old monster manuals depicted various undead such as "____ is the soul of a particularly nefarious over user of TPK monsters that has been transformed upon death into a Goo Spewer of Doom" or somesuch. No more waves of shadows wiping out entire villages, moving on to entire cities, etc., but just when one of these creatures goes through the whole death cycle. Keep the DAMAGE at d6, no save, but no death from it...treat it as all other damage is treated (well, except to con, I realize that).
Now here is a tid bit to keep you cranking....so if you get a bulls strength spell, str buff somehow and then a shadow damages you to death normally are these points untouched and thus you are immune past str 4? If not, would a belt of strength be different?