I have bought this product multiple times and been very happy, but my most recent conditioner smells terrible! I can’t use it! What can I do?

Send it back! I've found that when I buy it from Amazon about 50% of the time I get counterfit/old/BAD product. With this specific product it was AWFUL. Smelled horrible and made clumps of my hair fall out. Since I've experience that with this specific product, I've started buying it exclusively from companies that sell hair/beauty products, like Davines actual website, sephora, ulta, birchbox, etc. My advice is return it, and find a new retailer.

Hello: yes, this is sulfate and paraben-free.

Can I straighten my hair after using it? Or my hair will be so wavy? Thank you

Yes, you can straighten your hair after using it. I think the idea of the conditioner is that it hydrates curly hair, but doesn't make it curly.

Could I use this as a leave in conditioner, and not rinse it out?

The conditioner is thick, almost like a conditioning mask consistency. Leaving it in your hair would make your hair dull, I think. It's possible that if you dilute the conditioner with water you could use it as a leave-in, but if it were me (I have very dry hair) I'd rinse this conditioner out and use a separate leave-in. Good luck! Davines are nice products.