All the MP maps were ripped from campaing. You could argue that those spaces were designed with MP in mind but they all turned out be utter garbage for most gametypes.
No. They were made as MP maps first. In their entirety. Then they were integrated into campaign afterwards.
One of the main reasons was so that MP maps would be at the graphical level of campaign. That's why they were there, so that the art team could keep them at parity.
Reflection - used by dmiller to fill out his Alexandria level. Remake of Ivory Tower from Halo 2. A remake of Crater from ODST was also used as space in that level.
Countdown - you barely walked through this in campaign. It was used more like a skybox than playable space.
Zealot - the first part of the ship you enter in LNoS. You spend all of a minute in there.
Boardwalk - there isn't even gameplay on this space in campaign. It's just used as the intro space.
Powerhouse - you can skip it in 5 seconds, but it's separated by a canyon on one side and a gate on the other. It didn't have to integrate with the rest of the mission at all.
Spire - used as the last segment of Tip of the Spear. Barely any encounters in it.
Sword Base - used as the space between the last two major encounters of ONI.
Boneyard has the most gameplay/encounters on it, which makes sense considering that it was campaign first. It replaced an original MP map that had to be cut late in development.