Are other theme overrides working or is this your first modification? I’m unsure as to if the preview takes into account your theme, but I assume the theme is installed and active as this looks like it should work.
As for the other question of a hook…
Woocommerce emails work based on triggers you call do_action on the specific email hook you want to fire out, if you look in wp-content\plugins\woocommerce\includes\emails\class-wc-email-new-order.php you can see 6 triggers for new order emails, do claling do_action on one of these and passing an order Id would emulate a new order email.
The easiest way however would be to make a new order through the front/bac end with your email, then view the order, in the top right corner of the page under Order actions choose to resend the email you want then click but button on the right of the drop down (Not obvious that this is the button most people click save order which is wrong). This way you don’t have to make a page that fires a do_action trigger or anything. That and to do the do_action hook you need a order Id anyway.