All of your questions should be covered in the lease agreement. Normally, the landlord is responsible for making sure the parking area complies with ADA; however, you could get sued by a customer for not being handicap accessible. One thing you will definitely want to purchase is insurance covering ADA claims, given the issues you have identified on here.

1) The responsibility to ensure compliance with accessibility laws is jointly between landlord and tenant, but a lease agreement can delegate the party responsible for particular tasks. In common practice, the parking lot is the landlord's responsibility.
2) No one can really advise you where to place the handicap space without having seen it. Generally speaking, the location of the handicap accessible space must be reasonably close to the entrance to minimize the path of travel from the parking space to the entrance of the business.
3) Generally speaking, California law requires the wider van-accessible handicap parking space, as opposed to the regular handicap space under federal standards, unless there is some unique situation that would make it impossible or impractical to put in a van accessible space. A CASp inspector can help you determine what is readily achievable.
4) If the business facility provides parking for customers, you can restrict parking for customers only.
5) If the business does not provide public restrooms for customers, then the restroom for employees only does not have to be ADA compliant.

I strongly suggest that the landlord and/or tenant obtain a CASp inspection as soon as possible in order to take advantage of the protections under California's newly enacted SB 269.