(Ⅰ)由f(0)=2,解得:c=2,
∴f(x)=ax2+bx+2(a≠0),
由f(x+2)-f(x)
=[a(x+2)2+b(x+2)+2]-[ax2+bx+2]
=4ax+4a+2b
=16x,
∴
,解得:
4a=16 4a+2b=0
,
a=4 b=?8
∴f(x)=4x2-8x+2;
(Ⅱ)∵?x∈[1,2],使不等式f(x)>2x+m,
即?x∈[1,2],使不等式m<4x2-10x+2成立,
令g(x)=4x2-10x+2,x∈[1,2],
故g(x)最大=g(2)=-2,
∴m<-2.