Your original question has a lot of different facets to it. The first being the desire to start a school. When I first read your question, I thought you were going to ask advice on how to start a school. But as I read on, I feel that you are more asking about Krav certifications. Let me address what I think you are asking, from the perspective of someone who owned a dojo full time:
You do not need a certification to teach any art, whether it be Krav Maga, Kung Fu, TKD, etc. Certifications are given by the individual organizations that you are a part of, and there is no governing body requiring anything when it comes to teaching a particular art. It all is based on who you intend to market with. If I want to teach with and advertise with Krav Maga World Wide, then they want me to get a KMWW certification. It would be the same with KMG, etc. The general public wouldn't know the difference. You could even claim to be a Black Belt (even if you are not) and most of your students wouldn't know the difference, unless you are a really bad martial artist and instructor. I told this story on another thread, but I knew a Green Belt who opened a dojo and began wearing a Black Belt. His students didn't know the difference because he was a pretty good teacher. He went on to become a multiple-time sport karate champion and a hold many world records for breaking. He lied about being a Black Belt, but he grew into the rank and became a very good martial artist.
A lot of people open dojos without having any certification, other than maybe a Black Belt certificate. Teaching certifications are a great way for organizations to make money off of people who want to open dojos. That's not meant to sound insulting; it's actually a really smart way to make additional income. Many organizations have an "Instructor's College", but you don't need any certification if you are opening an independant dojo. I'll put it this way, as a six-year student of Krav, if I decide to open a Krav school I'm not going to spend money getting a certificate to teach the art; I'm just going to teach it. If I were to advertise that I am a KMWW dojo, then I would have to get their certificate. None of this applies to the quality of instruction, but to address your question about certificates, you do not need a certificate to teach an art or open a dojo. I am generalizing when I say this, but certificates are more for us - the experienced martial artists. The general public wouldn't know the difference.