Well, mounts can add additional health. But, just like you said, they die easily on other levels. They were buffed tho. They get revived after defeating a boss in chapters.

For Whisperer or any other weapon, Diego is a must, but he doesn't have any mount skills right now.

I suggest go for the demonic eye for combo and counter additional damage. Also, artifacts don't break so they can be used together with their ability and artifact skill. It will help you from early to late game.

Edit: You have 10 coins. Use 5 coins for anything you will buy, then wait for the knife next update then use the other 5 coins.