Regarding #1; Does this happen to all your previous Projects or perhaps only those Projects having extensions? What can it be? Without an image posted here of your Project blocks, we really don't know from the error message you received. Are you attempting to load a Project that you imported from Thunkable or AppyBuilder etc. Perhaps this is why the compiler cannot find a component.
Regarding #2, you might read the nb172a release notes
http://appinventor.mit.edu/ai2/nb172/ Simply, the App build replaces the previous App and GooglePlay builds. To build for Google Play,
use the App build ... Yes, this is again just like prior to August changes necessitated by Google's new GooglePlay security issues., AI2 has a single build process once again!
Regards,
Steve