Based on what we know currently about ten heartbeats, why does Szeth require ten heartbeats to bring forth his Honorblade?
Perception is a very important part of how these things all work, and remember the Honorblades work differently from everything else. Everything was based upon them. Why don't you read and find out what's going on there, but remember that the characters's perception is very important.
So then that's why at one point Shallan requires ten heartbeats and now she doesn't?
Right, it's the exact same reason that Kaladin's forehead wounds don't heal. Because he views himself as having those somewhere deep inside of him and he can't heal until that gets away. And it works for the same reason why in Warbreaker when you bring something to life, your intention rather than really what you say is what matters. It's all about perception.