Regarding your question about symbology, symbols are based on fonts, and the Esri fonts on which the ArcMap symbols are based hae been imported to ArcGIS Pro at this path:
C:\Program Files\ArcGIS\Pro\Resources\Fonts
If you have created custom symbols, or used fonts that were not originally installed with ArcMap, those symbols will draw with the system defaults when the MXDs are imported to ArcGIS Pro.
If ArcGIS Desktop is uninstalled from your computers you will not longer be able to open the ArcMap .MXD files in the native application but they can still be imported into ArcGIS Pro.