Asymptomatics are not a significant source for casual spread.
They may well be for close,intimate, contact though.
As in a family situation.
Have no source for that, just a general impression.
I cant answer that but I can tell you that while Patient 0 tested negative for whatever reason while laying in a hospital bed....they have assumed he was shedding the virus based on test results of which I dont remember what they were. New England Journal of Medicine
From what I have been reading the asymptomatics are not a significant source for causal spread for the first couple of days, then each day their spread potential increases...
I have not seen a study that quantifies how that works, exactly (doubling, tripling each day, whatever...).