1. developers get assigned by the maintainers/core-team so there is no duplication of effort, there are never multiple developers working on the same bounty. if there's no progress from the assignee, maintainers / other developers would check in and the issue would get reassigned, there is self-regulation. that being said, standardizing this process through a 'locking mechanism' is an interesting idea, we will think it through - thanks!
2. the sliding-scale fee model you are suggesting is an interesting idea, we'll think it through! if companies try bounties, are satisfied and would like to commit to a bounties budget, at the moment we offer the option to pre-pay fees at a discount. there's definitely room for experimentation here, thanks for your note!
3. great point, at the moment you'd evaluate people by looking at their github profiles and whether they've contributed to your project before (or other projects in your ecosystem). there's definitely room to improve that 'selection' process for maintainers. once again great point!