You can download Anniversary Maps to Halo: Reach but not to Anniversary.
there are three levels in halo CE anniversary
Yes. In fact, Halo: Reach was released a while ago and Anniversary was just recently released.
no
Is gravemind in Halo CE? There's your answer.
Yes, halo 4 and halo anniversary (which is a remake of halo combat evolved)
They have. it is Halo Anniversary, only on xbox 360
No
In Halo: Anniversary, you press the 'back' button on your controller to switch to the old graphics. It is left of the Xbox Guide Button. You can only do this in Halo: Anniversary.
there are no terminals in halo reach did you mean halo 3 or anniversary
Yes, there is a halo custom edition.
I have played and beat them both on all difficulties, both are challenging, however Halo Combat Evolved Anniversary is more challenging so that makes H.C.E.A. better than H.R.