What are the default metrics that are measured?

You can find the list of metrics tracked in the tool and their detailed definitions here.

Can there be custom metrics defined?

No. V1.0 doesn't support custom metrics to be defined. This feature is scoped into the 2021 roadmap.

If I change the default parameters while creating a strategy, do you have a what-if to show what the results were likely to be and the delta impact?

Parameters can be tweaked according to business requirements while creating a strategy - the results can be viewed in the site preview. You can always gauge the impact of a change using the A/B testing feature.