（1）若当$x\ge 0$时，$f\left(x\right)=1-x$，求$D\left(-1\right)$；

（2）若$f\left(x\right)$是奇函数，$f\left(x_1\right)\le f\left(x_2\right)$，且$x_1{x}_2\ne 0$，证明：$D\left(x_2\right)\subseteq D\left(x_1\right)$；

（3）设$f\left(x\right)$满足：①若$f\left(x_1\right)\le f\left(x_2\right)$，则$D\left(x_2\right)\subseteq D\left(x_1\right)$；②当$0<x<1$时，$f\left(x\right)<f\left(0\right)$．（i）证明：$f\left(0\right)\ge 1$；（ii）证明：$f\left(x\right)$在区间$\left(0,+\infty \right)$单调递增．