I bought my Thule Chariot Sport 2 version 2017 and we use this accessory for our baby.

In my case, the part that is shown in white in the product photo is grey.

According to the supplier, it should fit all Thule trailers.

There is different information on whether a helmet increases or reduces protection (additional forces acting on the head). We always rode without a helmet because the Chariot offers enough safety even with a headrest (we don't wear a helmet in the car either).

No, the combination of helmet and seat support does not work.

Unfortunately not, because the brand of the child seat is THULE and is not compatible with Burley Cub.

I only have the 1-seater trolley. I think 2 side by side work, yes.

The seat support can be fitted in all Thule/Chariot models.

We have another Chariot that basically fits two children. You could also use two seat supports there. The straps of the existing harnesses are threaded through the seat support. To be on the safe side, take a look at the video linked to the pictures. Then you can judge for yourself whether it would fit you.