See the video below. These are progressive options, so as you turn on the option to allow mapping AutoCAD Model Space to SE 2D Model View, then the Best Fit option is enabled and then you can turn it off to enable the manual scale options.
Nope, that is completely frozen. I am not able to make any changes on this side of the option sheet. Attached image.
According to Help, that option is only available IF the DWG file contains Model Space graphics. If the DWG does not, then it is Paper Space only and would have Viewports similar Solid Edge Drawing Views and thus will be imported as such.
"This option is only available if the AutoCAD file contains model space. If the AutoCAD file contains paper space, for example, view ports, the option is disabled and Solid Edge imports the file by creating 2D views to represent each view port."
Thank you very much. The only way to have a completed 1:1 scale in Solid Edge with this circumstance is to use the "scale" icon in "view".