"Now the metal body in the pullout section has cracked, and the unit sprays water at right angles to the faucet when the water us turned on."
It's too bad you wasted your money replacing the faucet, instead of getting replacement parts.
I e-mailed Grohe's Customer Service Department about the length of their warranty, and they replied "As long as you keep a copy of your receipt for the item, there would be a lifetime warranty."
To e-mail them, Google on: grohe customer service
The 1st Google hit is their site to send warranty claims.
If you can't find the receipt, but bought it via Amazon, you're in luck. Click on "Your Account", click "Your Orders", select the time period that you ordered the faucet, and click "Go". When you find your order, print it out and that's your receipt. I made a PDF of the receipt and sent it to Grohe and they accepted it. Good luck!