I would have to agree with your assessment that the Risk Surveys are not flexible enough to achieve this. The plugins tend to be pretty rigid in their functionality.
In theory, it might be possible to mess with the Risk objects to achieve functionality which is somewhat close to what you expect but this wouldn't be advised, supported or easy to achieve. Not to mention the functionality itself would not be intuitive.
Unfortunately, without some form of custom development, I don't think you will be able to achieve what your are looking for.
Simple answer is no, you can only have one risk questionnaire.
You could build out a new form with lots of questions and then use visibility and business rules to manage the flow and a calculation rule to work out the risk metrics. I expect this o be painful to configure or update in future but possible.
If you want to have something more flexible developed then please log it on the Ideas portal: https://ivantiitsm.uservoice.com/forums/904372-service-manager