Applying for postdocs and for Postdoctoral fellowships at the same time
A postdoc is not usually considered a permanent academic position. It is the most temporary of temp academia positions. The rule on not holding a &quot;permanent&quot; position likely means that you cannot be working as a (tenure-track) professor and then take the fellowship. This would be a question you would need to ask directly to the sponsors of the fellowship.
I also am not clear on how a postdoc and a Postdoctoral fellowship are different. These usually would be the same type of position in my mind. If so, the position at U of B versus the position with Prof X are really the same sort of thing. It would just come down to interests and pay at that point.