Re: Becoming an instructor
The minimum rank to be an certified instructor in the Aikikai (Fukushidoin) is second dan. That,s the closest to an official position as you'll get. Basically, there are no rules on who can start a dojo and teach.
Personnally I think it comes down to how much input from higher level people you can get. Second dan is not a bad level for a dojo cho if there isn't anybody more experienced around and he still maintains regular contact with higher ranking teachers. I would be skeptical of anybody working completely independently (no technical oversight and correction from above) unless he/she had reached shihan level (minimum rank of sixth dan in the Aikikai).
Of course the real tricky thing is that dan grade and skill level are only loosely correlated, especially when you start comparing between different dojos and/or organisations.