> Under UBI, how many people would willingly give up their jobs just to live on a subsistence level income, and for no other reason (e.g. study or childcare)?
Well the proportion will probably be smaller than the vast majority, skewed towards the asset rich when state pension ages kick in, but I don't think it'll be zero. Though even if it was zero, you've got the problem that under normal circumstances a full third of the population already doesn't work or seek benefits, and suddenly you've got to find the funds for them.
> Under means-tested benefits, how many people without work are discouraged from taking low-paid or part-time work because their social security payments are clawed back, resulting in no financial gain for them?
Relatively few even without competently-designed tapers and EITC schemes, bearing in mind the number of people receiving benefits contingent on unemployment is normally a relatively tiny fraction of the working age population.