Certain? No. Nobody can realistically be certain yet.

There is only one case that I've been able to find of a person who was diagnosed twice that can't easily be explained away as having been discharged while still infected. And even that case might be one like that.

We only know that you are likely to be immune for six month. Whether you will have life time immunity is anyone’s guess.

No. There might be even opposite effect - antibodies might make subsequent reinfections easier. That happened with some previous coronavirus. As well as with some vaccinations that didn't generate sufficient amount of antibodies - some kids acquired some illness much faster than non-vaccinated ones because the vaccine wasn't strong enough.

Current coronavirus has already multiple distinct strains. Even if they don't mutate further, we would need multiple vaccines. They might also mutate to something more deadly, given the COVID-19 is a mutation of the original SARS with some characteristics of HIV and Ebola.

We are in uncharted territory.