I hate you right now (not really). That makes entirely too much sense. I went back and re-read that bit again, and yeah... it fits.
10 years ago would put it around what, 1165? Shallan would've been nine? Also Adolin mentioned Bavland as a possibility, which is at least in Jah Keved. And if the deadeye was found on an island, that would imply a lake on the physical side, so it's possible someone threw the blade into a lake (that might be a bit of a stretch though).
Otherwise though, it always did seem odd that her father was able to take the blade and lock it away in a safe. After finding out about what shardblades really were I wondered why that would really do anything. But if it was a deadeye blade, then well that would be possible and it would stay.
Also, I remember Pattern being concerned about Shallan killing him, but I don't remember the exact wording though, or even where it was. Does anyone remember where that was? I It might be informative to re-read that section to see what exactly he said, and how he said it.