Try another dealer for a second opinion - also try sitting in an Avensis in the showroom and see if the seat behaves in the same way. Certainly a headrest rattle can't be any sort of a safety feature!

I like Toyota's but have had enough experience of them to know that their seats (apart from their niche sports cars) are a weak spot, both in design and longevity.

I'm a pretty generic shape, just under 6ft, 34" waist, not too broad shoulders, and can get comfortable in most modern cars, including Toyotas. However, longer term experience of Toyo's has not be so good. The seat in the last-gen RAV4 was rubbish I thought. Ditto the last Auris. Havent tried the Avensis yet, but looking at the pictures, it doesnt look like much to look forward to.