No it's not. Also, which estimates have been horribly off? I'm not aware of any that have been way off other than conservative estimates about hospital ventilator requirements (which would obviously be biased towards the worst case).
No, it's not - we know the ballpark value. See below.
No, it's not.
Also, see (I've posted these before):
Covid-19: Global summary
This is something that epidemiologists can figure out...
Of course it's malleable. And of course it depends on the particular country/region/group in question. That's why I was asking whether you thought masks would bring it to 1.