Re: formal "class" as a supplement to keiko?
I think you have an excellent idea. It is my understanding that Shigenobu Okumura did the same once a month at the Ueshiba's house next door to Honbu. He also did talk a lot during his keiko as well, going so far as to regularly using a chalkboard.
As for the problem of talking, I find that a lot of talking that goes on among people practicing is a defense mechanism against fear of practice. They are afraid and try to put off movement with talk. So to stop that, one must work to reduce the fear, then the talking will go down too.
As for the teacher talking, remember that O'sensei talked a lot.