Themes

I will try to answer your questions:

I added the Progress Completion block, but I'd like it to sit in the middle, stretched across the screen when a student is viewing course activities/resources. I made a row to hold a single block, but I can't drag the Progress block into it.

I understand you are talking about the course page. In that case just add the block in the top region and add the block there. See the demo site: https://adaptable.ws/demo/course/view.php?id=5  (login as student or teacher)

Is it possible to hide the right-side blocks and set the site to display in full screen, and hide the Show Blocks/Standard View options?

No. This is always a user setting. But you can change the default setting. Just search viewselect in the Administration block. A trick could be to hide the button from users once set viewselect to full screen.

Is there a way to send students directly to the first uncompleted activity when they open a course, instead of showing the activities list?

This is a course format question and not related to the theme. Try to find one that meet your requirements.