Good luck in court, if you even have that option. Losing will mean that you pick up the tab from the FAAs expensive lawyers and likely a stiffer penalty.Possibly. It is only a legal interpretation and not a regulation. The proper course of action if you disagree with it is to fly a UL that is in violation and ensure that the FAA gives you a citation. Then you will have the standing to take them to court and prove that it is "nonsense". Short of that you can just ignore the letter, fly like we have always done, and assume that the FAA will continue to show benign neglect toward fat ultralights.
As I see it, the letter is a correct interpretation of the current law. What we need is a concession somewhat like the float allowance. That won't happen by being combative. The FAA has no obligation to even provide 103, it's a low hassle compromise. Make it hassle and they may cease to see it as a good idea.
The best we can do is politely request extra weight for pure electric planes and hope that they throw us a 30lb bone.