Re: Homepage other than Activity or Admin Blog (or a WP page)
Success! Thank you, everyone!
Here’s what I did, following mercime’s advice.
1. Created a new page template page-home-tempate.php in the root of my child theme, with content
Template Name: Home Page
<?php locate_template( array( 'forums/index.php' ), true ); ?>
2. Created a new page in WPMU, called it ‘Home’, using that new page template.
3. Set that as my front page in Settings–>Reading
4. Deleted the Forums navigation item from header.php in my childtheme