Yes, you have to, it is prescribed by the manufacturer and ensures that the seat does not bend in the event of a collision. For these reasons, isofix stations, for example, have an additional foot. The problem of being trapped behind the front passenger is FIRE HAZARDOUS. Passenger seats lift forward even without a person in them and could bounce back with the energy generated. PLEASE DO NOT tinker and post "emergency solutions". There are reasons why manufacturers specify fixings!

The top tether would certainly be recommended. In the grandparents' car, which also does not have the anchor, we have mounted the child seat behind the passenger seat and pushed it all the way back. This way, the child seat is supported by the passenger seat in case of an accident. I hope this helps you :)