Re: Training in other schools of aikido
If it was a permanent ban and not just "I'd rather you didn't until you have a better basis here," I think I'd leave. I'd certainly think very hard about leaving.
It just doesn't seem like a healthy dynamic. It could come from insecurity in the teacher's abilities, or an excessive desire for control, or a history of infighting with other dojo, but none of those sound very good to me.
But it would be good to ask before quitting, just in case the answer is either "You need a more solid grounding first" or "I know the dojo you're considering and it's bad for reasons X, Y and Z." Or just maybe "All right, if you can do it without interfering with your training here"--it's hard to know till you try.