Question about Max employer contribution on solo 401k

3 quick questions.   I setup a solo 401k and wanted to confirm something.   I am personally listed as the employer and employee.  I know I can contribution $18k as the employee and 25% of income as the employer.  Questions.

1. The employer contribution (25% cap ) is based on my Net profit on my Schedule C after all deductions. Correct?

2. Both the combined Employee and Employer contributions are put on line 28 of 1040 together right?

3. I'm thinking of starting a S corp this year and paying myself that way. Do I need to have all new documents created for my solo 401k plan since the employer would be changing from myself (sole prop) to my sub chapter S corp?  Or can I just create an amendment to the plan docs?

Thanks for any input
Ray