Can ground wire be downsized mid run?
The answer is no if you're looking for a simple answer. 

More complex answer - What everyone is trying to say in the comments is that you can size the wire according to Table 250.122 in the NEC. This table allows you to size the equipment grounding conductors according to the breaker size protecting the circuit. Example - If your breaker is a 50A then you're equipment ground should be a #8 from beginning to end and there is no downsizing. Keep in mind that the NEC has a complete Article 250 Grounding and Bonded and is one of the most complicated Articles in the NEC.

In answer to part 3 of your question. There is no distance to floor for running PVC other than it should be installed in a correct and workmanship manner. Also it should be protected from any unnecessary damage. A full description is in Article 352 Rigid Polyvinyl Chloride Conduit Type PVC.