You will never be blindsided by #1. Because if the person finds it will be more complex than initial estimates they will tell you ASAP. If they don't, the problem is #4, not #1.

You -should- never be blindsided by #3. Because that's why daily standups exist. That's why tasking out a story. If the person is going down blind alleys, the -process- should be what brings that out, via daily standup or whatever. No amount of colocation will fix that, only explicit communication. You're not going to passively observe someone going down blind alleys.