Can I define eligibility, suitability or applicability conditions on the action and treatment levels?

You can define engagement policies for individual actions. For more information, see Defining action engagement policies.

Engagement policies are not available at treatment level. Actions convey an outcome or value to the customer, whereas treatments are the way in which the customer receives the action (for example, the specific web ad for the action). If needed, you can create multiple treatments for the same channel, and then allow the Next-Best-Action adaptive models to discover which treatment works best for which customers, without the use of treatment-specific engagement policies.

Can I define eligibility, suitability or applicability conditions on the issue level?

In Pega Marketing 8.3, this is not available.

Can I define eligibility, suitability or applicability conditions globally across all groups?

In Pega Marketing 8.3, this is not available.