4. Yes, that's the main point. A lot of people had been studying jujutsu, judo, kenjutsu, kendo, etc for a long time. Yet when they met Ueshiba, all that training didn't help them one little bit. Yet, somehow that training helped them to learn what Ueshiba was doing?
This may be off topic. But is there any proof that training with Ueshiba helped them in any way? Did they eventually win against Ueshiba? If not, how do you really know that training with him helped?
Lets break it down.
1) I'm a bad ass
2) I get my ass kicked by a bigger bad ass
3) I train with said bigger bad ass
4) Now we assume that training with said bad ass makes me even more bad ass?
This seems to have the assumption that just because a guy is awesome at fighting he is awesome at teaching. And thus would improve your ability to fight. We simply assume that training with Ueshiba helped him in (in terms of fighting) ways that training in other martial arts did not.