I want to force secondary rows using WP_List_Table

My recommendation is to structure the array so that your variations become “regular rows”.

I would use your implementation of prepare_items for that.

Here your array is changed to something like that:

    $this->items = array(
        array(
            'id'         => 2,
            'page'       => 'page2',
            'url'        => 'page2',
            'alt-id'     => null,
            'alt-title'  => null
        ),
        array(
            'id'         => null,
            'page'       => null,
            'url'        => null,
            'alt-id'     => 2,
            'alt-title'  => 'alternate-title-2'
        ),
        array(
            'id'         => null,
            'page'       => null,
            'url'        => null,
            'alt-id'     => 3,
            'alt-title'  => 'alternate-title-3'
        )
    );

Then I would define custom functions for all columns by using column_$custom( $item ) (docs: https://developer.wordpress.org/reference/classes/wp_list_table/).

Something like that:

function column_id($item){
    return is_null($item['id']) ? '' : intval($item['id']);
}

function column_page($item){
    return is_null($item['page']) ? '' : esc_html($item['page']);
}

function column_url($item){
    return is_null($item['url']) ? '' : esc_html($item['url']);
}

function column_alt_id($item){
    return is_null($item['alt-id']) ? '' : intval($item['alt-id']);
}

function column_alt_title($item){
    return is_null($item['alternate-title-3']) ? '' : esc_html($item['alternate-title-3']);
}

I assume you have already defined your columns with get_columns like that (or similar):

function get_columns() {
    return array(
        'id'        => 'ID',
        'page'      => 'Page',
        'url'       => 'URL',
        'alt_id'    => 'Alternative ID',
        'alt_title' => 'Alternative Title',
    );
}

This is key because your WP_List_Table subclass needs to know that you want to call your custom column methods (e.g. column_alt_title as shown above).