Earlier quoted context omitted.
Phase one is about dosage at least as much as safety. The most likely reason to fail is probably that it doesn't produce an immune response at any reasonable dosage.
Is there any risk of a "doesn't produce immune response" failure that would make it unsafe to start giving to healthcare workers immediately? Could it make later vaccines ineffective? I'm sure that it would ruin the data for later vaccines, but if high-risk people just take each new vaccine as it starts trials, their data doesn't have to count towards the actual trial.
A vaccine can make a disease worse by priming the immune system to overreact and attack the entire body when a single cell is infected.
The more we know about a vaccine the more confident we are. The flu vaccine is changed every year without long safety studies: we have learned enough about how people response to the flu vaccines in use to not need them anymore. When the same process is used to make a non-flu vaccine though we still have to start over because there isn't any way to be sure.
You are quickly getting beyond my area of knowledge though. I didn't answer all of your questions because I don't know.