First , thank you for such a big detailed post. A bit too technical even for me . I will try to add something to it not sure how successful i will be.

Well, windows reserved partition can be removed if you do not have Bitlocker feature with your windows 7. As you said, Reserved contains features for Bootfiles, Repair and BitLocker functionality (in Ultimate, Business & Enterprise).

Also, a disk can not have more than 4 primary partitions.

Regarding Virtual box, I never have used it but i am on VM which works almost like virtualbox. I am still to test more than 2 virtual OS in VMware but as far as i know we can have as many Virtual OS running inside a host OS with a VMware Virtual machine and I didnt find any problem running them. I cant say about Virtualbox as I have never tested it extensively.

As far as I can think of only one partition system reserved will be created even if you install multiple windows 7. I am just predicting it but someone else has to confirm it.