I can try to help to explain the first issue.
The general concept of Fusion is one of "direct modeling", or "non-parametric modeling", as opposed to traditional, parametric, history-based modeling. In non-parametric modeling, there is no model compute, no feature rollback, etc. All operations occur on the final state of the model. In Fusion, this model also extends to sketching. That is, Fusion does not capture sketch dimensions in a persistent manner. That's why there is no "Create Dimension" command in the sketch environment. Instead, Fusion applies temporary dimensions to sketch entities that are selected, while a sketch is active. These dimensions can be edited to precisely position/size the geometry. However, they are not permanent constraints on the model. The sketch geometry is updated, and subsequent changes may then cause those dimensions to be violated, or the user can drag sketch geometry to violate those dimensions. To keep sketch geometry in its current position, you can use the "lock/unlock geometry" command.
Jeff Strater (Fusion development)
Jeff Strater
Engineering Director