Also a lack of LinkedIn account makes you more suspicious and less likely to get hired. So this is additional value in having an account. For appearances.
That really depends on the field. Only one position asked about my LinkedIn. And that was because they had you apply via the site.
I didn't apply, because fuck that inside out.
Yeah I recently heard about people working multiple jobs at once - I wasn't surprised - with work from home being a thing and many jobs at big companies being not overly strenuous, you can get away with it.
A previous coworker had been not especially good at his job and left after two months, and a little later I went looking for his LinkedIn to see where he'd ended up. Couldn't find him but didn't give it much thought. A friend told me that he was working at a company up the street but was also working another job at the same time, and the penny dropped - you can't have LinkedIn and be working two jobs at once and reasonably expect to get away with it or get hired again.