Regarding second question:
In your example your adding user control (not an actual web part), that web cart is created based on. As long as inherited web part is based on the same code files (https://docs.kentico.com/display/K9/Creating+inherited+web+parts) in fact it is same user control, so you'll add it like the first one.
There might be some options depending on what is the difference between original and inherited web part.