"So are we saying the the "employee" deduction should be on Schedule 1, Part II, Line 16 and the "employer" portion should be on line 19, of Schedule C Profit and Loss From Business."
No. The deduction for both employee and employer contributions ends up combined on Schedule 1 line 16. The business-expense deduction for retirement contributions that goes on Schedule C is for employer contributions made to the accounts of non-owner employees (which you don't have because you are not permitted to have a solo 401(k) when you have eligible employees).
