2. For the most part, you just need someone who can do the job. Most people applying for seniors could; it's just a question of how fast/accurate/stable. For the most part, expected salaries tends to correlate with their ability.
Many of the bad hires I've seen were "cheap hires"; they applied for senior positions but were often paid half as much as a senior just because they were from a poorer country.
One guy had more ability as a researcher, less as a developer (he'd get things done very, very slowly, but produced well crafted code and was able to tackle the hardest problems). One guy was a great programmer but had personality issues, often getting into fights. Another scored highest on all the tests, but worked 12 hours/day and got nothing done. So I'm not sure assessments help in filtering most of these problems.