@Anhthu you can reference data from a task form later in the workflow, but I'm not 100% sure if that's what you're asking:
Regarding restarting workflows; you currently would need to restart the workflow from form submission. There is a feature request for the ability to restart a workflow that you could add your vote to here if you would like: https://nintex.uservoice.com/forums/430063-5-nintex-workflow-cloud/suggestions/32786005-the-ablity-to-rerun-a-workflow-especially-one-tha
Thanks. Hopefully that will be implemented not long from now.
And for my first question, to clarify, I was looking at a failed workflow instance and I saw in the logs how it captured the start form data input. However, I didn't see any data saved from subsequent forms submitted later in the workflow before it all failed. If that isn't done automatically, then I guess I would need to use the "Log to instance details" function then?