I'll be speaking to them in unofficial capacity soon. What all should we talk about?

The weather.

You are talking in an unofficial capacity I don't think any of your proposed questions should be discussed; except maybe showing interest in their future, did they already get another job? If not let them know you can be used as a reference.

What other things can I ask them to prevent/improve this situation in the future?

Don't ask, if they trust you they will let you know. May have nothing to do with your company at all.

Honestly, I think you need to ask yourself the questions you are proposing, if you are paying attention then you already know the answers.