I dont know the answer to your question, but I just wanna say that from what I read in OB where he can't say the 4th Ideal, I think it might be something along the lines of what you said ("I will forgive myself for those I fail to protect") It just makes sense to me. So, i like this theory.
I cant wait for when he swears it. He is gonna be so surprised when he gets Shardplate. its gonna be great.
Also, did you ask when he swears it? Or, if he will swear it?