Preview Widget button jumps back to the first page when clicked

I installed this widget to allow users to review the long-detailed form I designed.  I am able to "see" the PREVIEW button at end of form; other users see SUBMIT.  When they click SUBMIT, they are taken to beginning of form and then the see PREVIEW at end.  So this is sort of adding a step to their process.

How do I eliminate this?

I could not reproduce the problem. The preview before submit widget worked just fine on your form when I tested it here.

I think those are only user end errors. They might have skipped a required field when they switched between tabs and when they preview the form, it jumps back to the page with the required fields left unfilled.

Here's my test:

My suggestion would be to disallow switching of tabs when there are required fields unfilled. To do that, open your form tabs widget settings and set the following option "Allow navigation without validation" to NO.