Yes
yes
By getting halo 3 odst ( two discs ) first disc halo 3 odst campaign and 2nd disc halo 3 mythic comes with all the halo 3 maps.
No ODST comes as its own disc and if you own odst you get a beta of Halo Reach in 2010
Yes, there will be Forge mode in Halo 3: ODST... but on the other hand there is two discs so there might not be in the halo ODST disc.:)
Yes.
yes, unless you want to play halo 3 mythic which comes with ODST
It is not on a disc! It is on the sheet that says Halo Reach, But you will need Xbox Live.
yes, but its only multiplayer as in against each other
When you buy ODST it comes with a halo 3 matchmaking disc called mythic, it comes with the helm. Start> right stick tap right>apperance>armour> Helmet> ODST
Yes and no The achievements will be counted the same and so will the disks but if you have the ODST multiplayer disc (Halo 3 Mythic) and you join someone's campaighn on regular Halo 3 it won't work same thing if someone on HAlo 3 regular joins a map that comes with ODST for example SandBox.
halo 3 ODST will probably come with a second disc with all the Halo 3 Online maps on it (all 21) plus 3 more. will be halo 3 format.