如图,在三棱锥\(P-ABC\)中,\(PA⊥\)底面\(ABC\),\(∠BAC=90^{\circ}.\)点\(D\),\(E\),\(N\)分别为棱\(PA\),\(PC\),\(BC\)的中点,\(M\)是线段\(AD\)的中点,\(PA=AC=4\),\(AB=2\).
\((\)Ⅰ\()\)求证:\(MN/\!/\)平面\(BDE\);
\((\)Ⅱ\()\)求二面角\(C-EM-N\)的正弦值;
\((\)Ⅲ\()\)已知点\(H\)在棱\(PA\)上,且直线\(NH\)与直线\(BE\)所成角的余弦值为\( \dfrac {3 \sqrt {7}}{21}\),求线段\(AH\)的长.