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 the current version of Pega Customer Decision Hub, this is not available.

I want to show a new offer to the customers who interacted with another offer. Can I do this with an engagement policy?

Yes, you can configure an engagement policy to qualify customers for a new action if they interacted with another action before, for example, if the customers clicked on a mortgage offer or ignored a mortgage offer a number of times. First, you need to create a strategy that uses the interaction history to filter customers who have previously engaged with the action. Then, add a condition to your engagement policy that uses the results of that strategy to identify the customers who qualify for the new action.