Hello.
There are some plugins which promise such functionality however we haven’t tested any of them so we can’t safely recommend one, you can search for “woocommerce buy now button on variable products” and pick one. Keep in mind that in order for the customer to be able to select a variation, dropdown menus will appear above the buy now button, which might not look optimal depending on the theme you use.

Hello.
Try removing this line add_action( 'woocommerce_after_shop_loop_item', 'cssigniter_buy_now_button', 15 ); from the code, it should do the trick.

Hello.
Unfortunately due to the way WooCommerce handles variations, there is no simple way to do that. Functionality like that would require a more complex solution which falls outside of the scope of this post.

Hello.
This code should add the buy now button to the single product view for simple products.

Thank you