1) There is a hook for the first one:
You can use the apply_filters
Their code:
apply_filters( 'woocommerce_get_discounted_price', $price, $values, $this );
Your code:
add_filter( 'woocommerce_get_discounted_price', function($price, $values, $this)
{
//return whatever you want
},10,3
);
//*PHP 5.3 and up
you can pass whatever function you want into the second parameter of the add_filter function. Check out the Codex for details.
https://codex.wordpress.org/Function_Reference/add_filter
2) The second one will be very hacky and probably not recommended:
Though there are no hooks on the validate_cart_excluded_sale_items() function, the outcome is dependent up a transient value that you can override (if you are careful to put it back when you are done.
You can adjust the outcome of this functon: wc_get_product_ids_on_sale()
their code:
// Load from cache
$product_ids_on_sale = get_transient( 'wc_products_onsale' );
...
set_transient( 'wc_products_onsale', $product_ids_on_sale, DAY_IN_SECONDS * 30 );
what you could do:
store the current $product_ids_on_sale from the get_transient function
then set it to an empty array
set_transient( 'wc_products_onsale', [], DAY_IN_SECONDS * 30 );
then set it back when the validation has passed
set_transient( 'wc_products_onsale', $your_stored_product_ids_on_sale, DAY_IN_SECONDS * 30 );
AGAIN, I DO NOT RECOMMEND THIS, BUT I WANTED TO SHOW THAT IT IS POSSIBLE
It sounds like you should just get a better promotion for the sale