Does "Offline" in Disk Management actually turn off (spin down) the disk?
Setting a disk to Offline is basically a safe method of forcing a dismount if you cannot otherwise safely remove the device.
To access the disk again simply right click the offline disk and select Online.
As for power state the disks will not immediately spin down however since they are no longer being accessed the computer will turn them off after they time out. how long this takes can be set in your advanced power options under Turn Off Hard Disks. The disks will spin back up as soon as you Online them no need to restart
No, taking a disk offline just makes it unmountable it does not change the power state.
Can you clarify what you mean by if the logical disk is RAID0? Do you mean if the physical disk is part of a RAID array, will offlining the disk offline the whole array? No, if you have a dynamic disk setup in RAID0, offlining the disk will fail the array, if Windows will even still let you do that.
You should replace the drives with power saving drives, such as WD Green drives, if you're concerned about power usage.
Whether the disk spins down is determined solely by the disk firmware, not by your OS or any software installed.
Your OS tells the disk firmware after how much time of inactivity it should spin down -- through the setting Turn Off Hard Disks that you can change in the power configuration.
So just not accessing the disk is enough to let the firmware spin it down. To make sure that the (scheduled) OS services do not access your disk, you could take it offline, or remove the drive letter, or ...