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.
Don't downplay the safety. Phase 1 can fail because of lack of immune response OR safety concerns. Phase 2 and 3 trials are also about safety, and both of them somewhat often fail because of safety issues.
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.
Dengue fever is a strange and terrible disease where -- unlike most infectious diseases! -- contracting it once can make future infections worse through autoimmune mechanisms (if I remember correctly, you typically become immune to the particular strain that caused the first infection, but may become more susceptible to severe reactions from a later infection involving a different strain). Apparently the Dengvaxia vaccine, while effective at reducing the chance of dengue infections, also has an effect similar to surviving an infection: it increases the chance that an infection will result in severe disease. That makes it a more complicated question whether it's a good health intervention in particular circumstances, and in the Philippines there was a big scandal where a childhood immunization program using it may have reduced the total number of people who contracted dengue, but also increased the severity of the worst cases.
To my knowledge the pathogens that cause COVID-19 and dengue fever are extremely different, but it seems logically possible that there could be a COVID vaccine that greatly reduced people's chance of getting sick at all, while also increasing the chances of severe symptoms for those people who did get sick.
