1. These are not your taxpayer client's records. 1099-K is from a payment settlement entity. If your client runs customer payments through Venmo or PayPal, that is duplicated data from 1099-NEC or Misc in many cases. If your taxpayer has any private citizens as customers, there would never be any 1099-NEC or MISC issued, because private citizens don't issue them. Only business to business activities result in issuing 1099-NEC and that's issued when the annual amount has reached at least $600.

That means your client might make $3 million in business and never get any 1099. Would you then put $0 on the tax form? Of course not. You need the client's full records. You review the documents issued as part of due diligence.

2. Single Member LLC = disregarded entity. One operation, one Sched C.

4. Your client needs counseling when they form a corporation. They will fill in a W9 and I like to include a letter for "we are now doing business as XYZ S Corp" and your client signs as President. And now your client has to be running payroll.