已知向量\(\overrightarrow{m}=\left(1,1\right) \),向量\(\overrightarrow{n} \)与向量\(\overrightarrow{m}\)夹角为\(\dfrac{3}{4}π \),且\(\overrightarrow{m}· \overrightarrow{n}=-1 \).
\((1)\)若向量\(\overrightarrow{n} \)与向量\(\overrightarrow{q} =(1,0)\)的夹角为\(\dfrac{π}{2} \),向量\(\overrightarrow{p}=\left(\cos A,2\cos \dfrac{2C}{2}\right) \),其中\(A\),\(C\)为\(\triangle ABC\)的内角,且\(A\),\(B\),\(C\)依次成等差数列,试求\(|\overrightarrow{n} +\overrightarrow{p} |\)的取值范围.
\((2)\)若\(A\)、\(B\)、\(C\)为\(\triangle ABC\)的内角,且\(A\),\(B\),\(C\)依次成等差数列,\(A\leqslant B\leqslant C\),设\(f(A)=\sin 2A-2(\sin A+\cos A)+a^{2}\),\(f(A)\)的最大值为\(5-2 \sqrt{2} \),关于\(x\)的方程\(\sin (ax+\dfrac{\pi }{3})=\dfrac{m}{2}\)在\(\left[0, \dfrac{π}{2}\right] \)上有相异实根,求\(m\)的取值范围.