I boot from a software RAID partition, in a manner similar to yours. One thing I learnt the hard way was that
/var needed to be on the same logical partition as
/usr/lib probably did, too.
My RAID is configured in the BIOS with Intel Matrix Storage Manager. The RAID device has two partitions; one NTFS and one ext4, for dual-booting into Windows and Linux. I try to access the Windows partition as little as possible when running Linux, and vice-versa. The only problem I've ever had with it was a BSOD in windows with an error message about ntfs.sys. Long story short, I had to rebuild one of the my hard drives and reinstall Windows...
Installing Ubuntu onto the partition initially was a bit of a problem, as I didn't have an Ubuntu installation CD which could recognise the RAID device without first installing extra software (
dmraid). So I booted into a Live CD, installed
dmraid,and then installed Ubuntu on to the RAID using the debootstrap / chroot method. It's been fine ever since (over 2 years now). I don't have LVM or mdm installed.
In hindsight, if I were to rebuild my system from scratch, with your number of hard drives and no need for Windows, I'd almost definitely use ZFS instead of Intel's RAID manager.
Rebuilding software RAID arrays is done in the OS, so in my case I had to get familiar with using
dmraid on the command-line. It was a bit scary, to be frank.