Is running FlexRaid on Amahi going to be a problem?
If not, I'd be happy to try it out and to update the wiki.
Quite honestly I never used it. The guidance may be close, just has not been tested on Amahi 7. Like everything else, you have to try it to know for sure. If you are willing, recommend that's the first thing you do before spending hours getting your server set up to find out you hosed it.
So hopefully if the Amahi appliance is sleeping, they just won't need DNS and DHCP?
If you don't need connection, then it should be fine. Once it wakes up, clients can then get their DHCP lease from the HDA. This may take some testing to see if it will work as you expect.
I haven't seen anyone discuss whether Greyhole spins up all the disks when a file is requested, or just the disk that the file is on, though I've googled this etc.
Not sure if this question has ever been asked or answered before. If you access files directly on the HDA, change them, then it will most likely sense it and spin up the disks. I am no expert on this one, so you might try asking the Greyhole author. His web site should provide for this sort of questioning.
A lot of what you seek should be possible with Amahi. Unfortunately there is no guarantee it will meet you expectations without testing.