Still under investigation, but:

double normalize(double d, int& exp)
{
	if(IsNull(d) || d == 0)
	{
		exp = Null;
		return d;
	}
	bool sign = (d < 0);
	if(sign) d = -d;
	exp = minmax<int>(ilog10(d), -308, +308); // 8-byte double!
	d /= ipow10(exp);
	if(d >= 10) { d /= 10; exp++; }
	if(d < 1)   { d *= 10; exp--; }
	return sign ? -d : d;
}

Looks like fixes the problem. This, and adding special case to suppress the inherent U++ IsNull mechanic. And some custom work to handle infinite better.