The on-going Phase 2/3 trials in the UK, South Africa and Brazil are designed to establish the efficacy of the vaccine under viral challenge. The idea is that you vaccinate a bunch of people (and an equally-sized control group) and wait around until a subset of them test positive for the virus. Then you unblind the groups and make sure that the people who got sick were in the control group. You also check for ADE (antibody dependent enhancement) to make sure the vaccine doesn't amplify the strength of the virus.
All your questions are valid, but this study doesn't claim to answer them. Those results are still forthcoming. Additionally, the risk of ADE is well understood and carefully considered in the design of these trials.
(Disclaimer - I'm a participant in the UK Phase 2/3 trial)