some valid points, but also some that are specific to your region, the products and regulations are different here in the UK
The seats that use just a seatbelt to attach, are just as safe here, and also on Isofix, not all seats require the use of the third tether.
The best advice is to go to a specialist retailer who will allow you to try the seats in your MINI, and find which one you find easiest to use and which best fits your car, as all cars can differ slightly from the next, even in the same model.