To answer each of your questions:
1. In general, amending 2017 requires using 2017 forms. TurboTax will use the correct Form 1040-X for 2017. The main purpose of Form 1040-X is to report what was originally filed, report the change, and report the changed amounts. See: How do I amend my 2017 TurboTax return?
2. If Form 1099-B is necessary, yes, include it because it is the reason for the amended return. See Instructions for Form 1040-X page 22, first column: Assemble any schedules and forms behind Form 1040-X in the order of the “Attachment Sequence No.” shown in the upper-right corner of the schedule or form. If you have supporting statements, arrange them in the same order as the schedules or forms they support and attach them last.
3. No. Form 1040-X instructions say only to sign Form 1040-X. It also states to not include the originally filed Form 1040. There would be no other forms requiring signature.
@_confused_
