Sadly, that's about mounting a raid within an already booted system. We know the livecd can do that or it wouldn't be able to install to your drives. The problem seems to be in the mkinitrd script which creates a temporary file system for the kernel to use while booting, before it mounts the real partitions. This gives it access to drivers it needs to access the real partitions, I think (not quite clear why this two step process is needed). To work, the initial ram disc (initrd.img for short) needs content identical to the real stuff so the kernel doesn't notice when it changes over, so it has to be made on the machine during installation to match what's actually there.
The script which does this doesn't cope with RAID very well because there are too many variations, some of which are bodges for MS which are non-standard and difficult to know about, which means it is difficult to get a kernel to boot from a RAID. It's almost there, but not quite.
O-P's workaround of putting a /boot partition on a USB stick would probably make things possible. As grub appears to be working, you could probably still install the boot loader to the MBR of the RAID, although there are issues to be considered:
1. With Grub on the USB stick you would have to change the BIOS settings to boot from that, but you could use a Windows disc to restore the Windows boot loader to the RAID MBR. That would mean the machine could boot Windows in the standard way if the USB stick were removed. On the other hand, until you do that, Windows could only boot when the stick is present.
2. With Grub on the RAID you would not have to change the BIOS settings, providing it could find the USB stick. Windows would be booted by Grub even if the USB stick were removed.
Which you prefer would be your decision.
At least it would get you going.