It is all about line of effect. You can detect someone as long as you could, for instance, throw a rock at them without it being interrupted.
The discussion of visibility and concealment just means that a blind creature is blind. Blind sense doesn't give you senses on par with DareDevil (with 3D echolocation based vision). It is basically like...an advanced form of scent really (and heck, it might be scent based), where you do not need to waste time with all that concentration. You can tell where things are, but you can't tell if he is dodging to the left or right, or anything precise like that.
So
1. Yes. You can't see. You are going to miss like normal.
2. You detect them, but the mist still has normal concealment for hit purposes.
3. No. No line of sight. The sense you need for burrowing is tremor sense (which gets you about the same degree of info). And as far as I am aware, you need to be in contact with the ground for tremor sense to work.
4. Not unless the dragon has tremorsense.
Now, some of the disadvantages of 'can't actually see' can be alleviated with normal blindfight feats.