multiple activities affected by a single risk

Definitively resource availability might not be deterministic, this is an excellent question.

If using Spider Project I would suggest using the 3 scenarios approach to model different resource availability profiles [optimistic, pessimistic and probable] using Time or Activity resource production. 

This method does not have many of the limitations Monte Carlo models do have and can be implemented using your CPM software without the need for specialized Risk software.

http://www.spiderproject.com/images/img/pdf/Methods%20and%20Tools%20of%20Success%20Driven%20Project%20Management.pdf

The following SPERT Excel Templates might help you get the distribution curves, with little modification you can get the cumulative % distributions.  Statistical PERT Normal Edition uses ratio scale multipliers to modify the standard deviation of a normal curve. This lets Statistical PERT work with any bell-shaped uncertainty that can be estimated using a 3-point estimate for minimum, maximum and most likely outcomes.

https://www.statisticalpert.com/download-free-templates/#duration

Hi Mark - Welcome to Planning Planet

There are many types of risk that affect multiple tasks - weather is typicle.

If there is likely to be a shortgae of labour then put a cap on the number expected to be deployed and the end date will extend accordingly.

Best regards

Mike Testro