No, a workflow assignee cannot be substituted with another user.
No, if a member of the workflow is not able to complete their step, it is recommended that you invite an additional assignee to the workflow. See Add Additional Assignees to a Workflow Step on a Project.
No, if the workflow template needs to be changed, the object would need to be deleted and recreated after setting a different default template.
Custom statuses are not supported by workflows. Workflow step names and responses can be customized but the associated status choices come from the item itself.
No, there can only be one default workflow applied at a time. The Commitments tool is an exception as it allows a default workflow to be set for subcontracts and purchase orders separately.
No, but you have the option in the Workflows tool have specific workflow templates automatically added to all new projects.
No, an attachment added to a specific workflow response does not carry forward to the following steps.
Yes, when looking at an item with a workflow you can view the workflow by clicking on the vertical ellipsis in the workflow side panel.
Calendar days are each day of the month including weekends and business days are only Monday through Friday. Business days are localized to a project's time zone and are not related to 'working days', another Procore feature. See Set Project Working Days