Add to cart has been translated into the WooCommerce language files and Buy Now has been translated into the Theme language files.
2- You can change the icon using custom CSS code only. Go to the Dashboard >> Theme Settings >> Theme Custom CSS >> Global CSS and then paste the below code at the bottom of the CSS box.
Then save settings and check back to your issue after clearing the browser cache.
Note: In the above code the word (\e916) is the icon code you can change it with the icon code you want. There icons can be used – https://xstore.8theme.com/xstore-icons/
3 & 4- You can change the add to cart and buy now buttons colors from the Theme settings >> WooCommerce >> Single Product Builder >> Add to Cart & Quantity see the image: https://postimg.cc/ZWtWDjTj and for hover see the image: https://postimg.cc/64CX7rD5
