No, once configured on creation, the property cannot be changed anymore. You will need to re-create the VD if you wish to change this property. 
Depends on how much data you store in a volume with fast rebuild enabled, the more empty the volume is, the faster the rebuilding speed is. If the volume is fully occupied then the fast rebuild does not bring any improvement in rebuilding time. 
By design a thin-provisioned RG already rebuilds it's VDs in a fast-rebuild manner by defualt and cannot be changed, therefore the option is not availeb for adjusting. 
The fast rebuild is a feature that eliminats the time required for rebuilding a virtual disk. 
Traditionally without fast rebuild if a virtual disk needs to be rebuit, the rebuild process rebuilds every single block of the virtual disk space no matter the blocks are empty or filled with data.
With fast rebuild enabled for a VD, the rebuild process then only rebuild the blocks with data and leave the empty space untouched.
No, the SSD cahce only stores a cached copy of the data that is actually existing on your VD that is assigned with the SSD cahce disk. If the SSD cache space stops working during operation then the access turns to the HDD space to obtain the data. 
You may estimate the size of data that is to be randomly accessed on the VD to decide the size of SSD read cache to be deployed to the VD.
There are a few factors to consider:
1. The access pattern: The SSD read cache is mainly designed and implemented to raise the random access speed of spinning disk arrays. Therefore if the access pattern of your application is mostly sequential, using SSD cache would not bring much benefits.
2. The SSD read cache needs warm-up time. If the VD has not been randomly accessed much after the SSD cache assignment, then it's expected that the data has not been populated much into the SSD read cache yet, so that the performance does not have a obvious increase. 
A SSD cache disk is assigned to a VD, in a 1 to 1 relationship, which means once assigned, the SSD disk becomes a dedicated cache space for the VD and cannot be used by any other VDs.
No, only SSD read cache is supported on Qsan SAN systems. 
Currently you may configure up to 2TB SSD cache space on a Qsan SAN system. The number and size of disks is not limited, and therefore you may use various combination of SSD cache disks on one system.