I feel reasonably confident about vaccine efficacy, I suspect the reason this thing is so bad is because of the binding affinity of its spike protein. If that mutates, it will become less lethal so waning vaccine efficacy becomes less of an issue.

Well, even if it mutates in a way that is beneficial (to us), you still have the original strain which will continue to spread unabated without a vaccine. It was just a handful of people who were infective initially, and now the diseases has spread everywhere. It can still happen again.

Also, for diseases that are considered "eradicated" because we now have a vaccine, if you are one of the insane anti-vax people and stop giving vaccines to your kids you can still have outbreaks because of natural reservoirs for the disease. (For e.g. measles outbreak in the state of Washington). Infectious diseases rarely just go away completely.

Any idea whether a COVID-19 vaccine can be added to the annual flu shot cocktail or are we going to be asking folks to get vaccinated twice each year? If 2 vaccines, can they be given at the same time or would we need to space them out for a period?

Probably too soon to know for certain but could add to the challenges of attaining and maintaining herd immunity.

There are multiple COVID vaccines in play, and they use different technologies. Assuming you have a choice, it would depend on which of those you're administering. But yes, it is certainly possible. There already exist combination vaccines like MMR, DPT, etc