1 .sure but we are also humans and can make mistake , there are several use cases need to be taken care of , i was missing one of the many scenarios thats why it took these much attempts , lastly I shared tested code .
simple to solve just edit the course and disable "Full units in curriculum ".
2. yes this loading bar indicates the quiz is being submitting and yes I agrre there is some flaw with the UI/UX of it .but its related to old plugin .
we can hide it because the loader already shows ,please add this custom css in your wp-admin -> appearance -> custom css :
.submit_quiz_progress {
display: none;
}