@captjames747 You're not the only one! You need to run the AIRAC installer found in the ......\X-Plane 11\Aircraft\JF_BAe_146\plugins folder. This should really have been highlighted as part of the update's download or installation process.
VNAV can be engaged at/above 400ft AGL by pressing the VNAV mode selector. Yes you're correct. The MCP altitude will effectively over-ride a VNAV descent until you move it to a lower altitude. The MCP altitude is often set to the first altitude restriction on the STAR at this point.