I think Tesla has a distinct advantage over Waymo because of their ability to control the entire vehicle stack from low-level ECUs to the batteries, motors and high-level autonomous stack.
Waymo has to partner with automobile OEMs who are still figuring out how to build electric vehicles, and build them in a way that can integrate with Waymo's technology.
I don't think there is any reason for Waymo not to pair up their system with a regular petrol car. Electric vehicle capability is orthogonal to self driving.
Depends, if they are planning to run a taxi fleet or restrict themselves to new personal vehicles, integration and operational costs for ICE vehicles is likely going to be higher than EVs in a couple of years. There is little common between the vehicle dynamics and control for ICE and Electric vehicles, and it might be more practical to focus on one platform.
Waymo has to partner with automobile OEMs who are still figuring out how to build electric vehicles, and build them in a way that can integrate with Waymo's technology.