I fear you will be really disappointed with the heating results if you install the pump as you indicate.
I think you are expecting too much from that pump to adequately heat the downstairs area. Base on our experience with the cooling ability of our pump, if we were intending to stay in our current house long term we would be installing a second pump upstairs.
Another factor to consider is even though you can blow the heated air downwards the heated air will still rise. Also the air being drawn into the heat pump will be warm air rather than cold or colder air if it were mounted lower. Having a heat pump mounted lower rather than higher for heating is a very effective way of mixing all the air in the room and warming the colder air. The converse applies for when the cooling mode is used.
I think you may need to consider two pumps or a ducted system to gain the desired results.
One other thing to consider is the draught created by the airflow from the heat pump. This can be quite annoying for many people, careful location of the pump can minimise the effects of this.
I am not a heat pump expert, my comments are purely based on personal experience. I would urge you to get some good advice on the best location prior to hitting the Go button.
You make some good points.
Do you feel any different about it if I point out that the ceiling slopes up from the windows to the bedroom upstairs? I'm gonna take a stab and say maybe it slope up 400-600mm from the window edge to the opposite edge of the bedroom.