The one thing I haven't noticed anyone mention yet is that when spren die their shardeblades are stuck in the phsyical world. It wasn't until the invention of attaching a ruby gemstone to the base of the shardblades were they finally able to be bonded and dismissed to shadesmar. So Shallans testament blade should be stuck in the real world until someone takes the blade and gets a gemstone attached to it.
The other issue that most people are glossing over is that if shallans testament blade as a child is small, obviously they have variable sizes but at the point she broke her bond the blade would have been stuck in that small form she had summoned it in. she's clearly using a larger sized and variable sized blade every other time she summons one so its obviously pattern, a living blade.
