If you want to mount 3 sites next to each other, you must first check with your car whether it is permitted. I recommend the following website for this: https://www.adac.de/rund-ums-fahrzeug/ausstattung-technik-zubehoer/kindersitze/kindersitzberater/drei-kindersitze-rueckbank/ Therefore, it has less to do with the seat than with the vehicle. Otherwise, you can fasten the seat in the centre with the belt, provided the child is already the right size.

https://youtu.be/7JF5yz13-BA

You can just use a seat belt to install this. They have YouTube videos on how to do it. Just look up brand and model.

It has a hinged flap under the seat. When you put it up, the seat surface is slanted. The backrest cannot be adjusted. For this reason we sold the seat and decided to buy another model.

According to own research, the width of 46.5 cm has been calculated with the side impact protection.

Unfortunately not. https://www.tcs.ch/fr/tests-conseils/tests/sieges-enfants/

Unfortunately, you cannot adjust the length of the Isofix arms on the seat. This is due to the joint on the Isofix arms. The seat can optionally be mounted either with or without the Isofix bracket.