" it's asking for the fair market value of the vehicle/total amount spent on the lease payments "
You will enter the fair market value if you OWN the car OR the lease payments if you lease it but you do NOT enter both.
You are mixing up terms ... taking the standard mileage rate on the Sch C as a deduction had nothing to do with taking the standard deduction on the form 1040 instead of the Sch A itemized deduction.
No... again a term confusion .... for the car ...if you take the ACTUAL expenses option instead of the Standard Mileage rate you will need to use ONLY the Actual expenses method for the life of the lease. You will not be able to switch to the standard mileage rate in the future. This still has nothing to do with being able to itemize deductions on the Sch A.