I believe that is right. The only persons I know of from the book who went in the flesh without a gateway were Slayer and Perrin. Even Rand and Rhavin used a Gateway.
I think yes. The dreamwalkers tell Egwene that she must not use that method. Slayer learned the method and he was not a dreamwalker or wolf-brother. I believe it is possible, I am curious on the negative effects of it. They never really went into what happened. Maybe, they become Aelfin or...