All very solid.
I would tend to assume the radios might fail. Conservative - perhaps, but as ZA says it is straight forward when you can maintain radio contact but things are more likely to go wrong if you cant, so plan accordingly.
If you are operating in anything like reasonable viz realistically you are only likely to loose company because the viz has diminished substantially. If this were the case I would want a greater margin between heights. After all there is a good chance you will not "form up" again and it will become two seperate flights. Moreover you would not want any confusion over altimeter settings - albeit that would have been discussed at the pre flight.
In the terms of the FP, I assume you are referring to an international flight requiring a FP. Use box 9 accordingly. The fact that you are forming up shortly after departure and breaking up shortly before arrival is incidental if you are intending to fly in company for the route. If you break up en route tell AT that G-XXXX flying in company with G-XXX2 is now no longer in company but will be continuing in accordance with the original FP.