Infant in seat fares are generally 10% to 25% of the adult fare - the infant fare is based on cabin , so book business class and pay percentage of the business class adult fare
EUR450 doesn't seem that bad
Changing to QR and you are likely to run into difficulties actually getting a ticket issued. If EY will issue it, then I would suggest taking it
You are going to need to pay fare based on journey being undertaken by the accompanying adult; if you book the adult in economy, you will have to book the infant in economy.