You need a driving licence that is valid in Austria and that you've held for at least a year, and you must be at least 21. A foreign licence must be valid in Austria.
Yes, as an extra driver. They must be at least 21 and have held a licence valid in Austria for at least a year. Their licence number also has to be on record. Anyone who isn't registered as an extra driver mustn't drive, not even for a short trip.
You pay for fuel, tolls (the motorway vignette is already on the car), ferry fares and fines, including parking fines. If you drive more kilometres than are included, you pay for those on top. Insurance, 24/7 roadside assistance and the service fee are already in the price.
You request an extension in the GoMore app once your booking has started. It only counts if the owner agrees to it. If they don't and you return the car late, you pay nothing for up to 15 minutes, and above that 0.7% of the daily price per minute from the first minute. From 24 hours, for each day or part of a day, the daily price and a €65 admin fee are added. The details are in the help centre.
Yes, if you add the insurance for driving abroad when you book. You'll see what it costs when you book. If you forgot, you can add it later, up to the start of the rental, by editing your booking. Without it you may not drive abroad. If you do anyway, you pay €350. Everything is in the help centre.
You pay online through GoMore. For cars without instant booking you send a request, and the payment is only taken once the owner accepts it. With an instant booking, your booking is confirmed straight away. Never pay outside GoMore, not even for delivery.