No, halo 3 forge is not offline, you may play it online with friends or others.
no... you can only use it to talk to other players whilst in matchmaking
You can get 43 achievements offline on halo 3 im not sure on odst.
It depends, if you are asking about using mics on halo 3 on xbox live, well then yes and no. If you have two xbox live accounts on the same console and each account is using a mic, then yes. But if you have one xbox live account on a console, only the person using that account will be able to use a mic. Guests in halo 3 are not allowed to use mics at anytime. Only users with xbox live accounts can use mics.
Yes, Halo 3: ODST will feature offline cooperative Firefight mode.
You can if you are playing offline or a custom game, but skulls are removed from the maps in matchmaking
Up to 16 with system-link, but only 4 with split screen.
You can use a tape recorder to record a conversation on Halo 3.
No, this is only a feature for Halo : Reach
its all about making you buy an xbox live card. To play online costs money and Microsoft and Bungie like money. I do agree that is unfair but playing online is worth the money.
finding Death Cards unlocks certain "Perks" in online gameplay. Similar to the Halo 3 skulls (If you know what I'm talking about.)
use the forge feature in any game after halo 3. In Halo CE, there is no way to do it.