1. Sure, use discount type Percent. That will only apply discount to the matching products.
2. Use this snippet:
add_filter( 'woocommerce_cart_item_subtotal', function( $subtotal, $cart_item, $cart_item_key ){
if ( $cart_item['line_total'] < $cart_item['line_subtotal'] ) {
if ( WC()->cart->display_prices_including_tax() ) {
$subtotal = sprintf( '<s>%s</s> %s', wc_price( $cart_item['line_subtotal'] + $cart_item['line_subtotal_tax'] ), wc_price( $cart_item['line_total'] + $cart_item['line_tax'] ) );
} else {
$subtotal = sprintf( '<s>%s</s> %s', wc_price( $cart_item['line_subtotal'] ), wc_price( $cart_item['line_total'] ) );
}
}
return $subtotal;
}, 20, 3 );