You're welcome and thank you for the feedback.

Correct. Keep the standard 5-day calendar as the project calendar and as the base calendar for the resources. The 7-day calendar is only used for the calculation of the custom duration field.

You cannot have the custom field be both editable and calculated by the formula. However, if the duration changes the formula will automatically recalculate.

You cannot rename the standard Duration field. You may only rename custom fields.