1. If users are NOT aware of being tracked, it is HELL creepy  
2. Scalability is now not an issue :) I think your concern should be to find the first customer.  
3. not sure  
4. I think they would, but you need to make sure users are not creeped out by it.  
5. Unsure. You might want to try both ways. Start marketing a specific use-case and see what happens (if it works, etc..).