You can adjust any of the BBC settings shown in the loading screen. During the loading screen, quickly press and hold any of the action keys until the player has spawned. AI Team Play is disabled by default. (Controller players; MCC has a bug that requires MCC to be in focus with a keyboard and mouse before playing with a controller/gamepad. Usually clicking anywhere within the menus should "reconnect" the controller state between BLAM and UE4 for this feature to work properly.)

Yes. In Custom Multiplayer, you will experience the raw state with these bots. Killing an AI in multiplayer does not contribute towards Slayer points. (Bungie did not plan for this feature to exist including later titles).

Yes. In campaign, Bots are designed to run the objectives as a player would, where this would not be possible in multiplayer.

Yes. With other mods you and a friend on the couch can experience Bot Boot Camp CO-OP.
However, LOCAL machines running up to 4 player splitscreen is the only work around with netcode related issues and AI.

Halo's difficulty settings have been replaced allowing full control of your party-size. Legendary mode only allows story progression throughout the campaign. Legendary will also enable Easter Eggs for you to find along the way.

No. Downloading mods will operate just the same as any title in MCC would. It's by design that you'll be automatically redirected to Install the missing contents.