We're not sure we've understood your question, but if you need us to, we'll be happy to help, by e-mail, chat or phone: https://helpcenter.galaxus.ch/hc/fr/requests/new

This model is the X2 Fix. The manufacturer does not indicate that the backrest is detachable, so we do not believe this is possible.

a child seat is a safety product, vital. you don't know what the other person will do with it in those 30 days. do you want to rely on the word of another person for the life of your child? would you buy a used child seat?

I would never recommend fitting the side impact protection on one side. Unfortunately, I could not find any information on the other questions. This is also not stated on the manufacturer's website: https://cybex-online.com/de-de/car-seats/solutionx2fix/1 Maybe it is worth asking the support (manufacturer or Galaxus) here.