WP_Query filter and order by meta ordering by wrong joined table

^{Note: Only read briefly, so I might missunderstood your Q.}

JOIN on meta_query/post_meta table is pretty … hm, funny(?) … as you’ll JOIN the table (again) for every key you add – this is a known problem (read on trac). In detail INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id) is the same as INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id). And ORDER BY wp_postmeta.meta_value ASC is the same as ORDER BY mt1.meta_value ASC.

You could try using the posts_clauses or posts_orderby filter to change the ORDER BY statement:

function wpse_change_orderby( $orderby )
{
    return 'ORDER BY DATE ASC';
}
add_filter( 'posts_orderby', 'wpse_change_orderby' );