Sonyalpharumors.com seems to just be a collection of various online rumours. Substantiated or not. Not that I don't appreciate the effort they put into gathering such information from the vastness of the web, but they do live up to the "rumour" aspect of their name. Bjorn on the other hand has a source that seems to be in the know at Sony, and when he gives details on something, its usually a sure thing. In fact, I believe the operator for Alpharumours relies on posts by Bjorn as credible information.
Yes, but they do rate their rumor from 1 to 5 as to how credible they believe their source is. If the rumor comes true, they boost the ranking for that source for future. They predicted A450 months before anyone else had any clue. They also dig up links to lot of interesting reviews, news etc. It's an interesting site and updated almost daily.
As for mirrorless cameras, they had more than just dyxym member (who is Bjorn source too). They were posting another Chinese source also.