The first link you referenced really, REALLY glosses over what MS chicken is. The only thing that is questionable in the "circulated e-mail" they reference is the "immature sex glands" part. And that is doubtful to me because I have no doubt in my mind the sex glands get used in other products first. Mechanical separation is the meat processing equivalent to strip-mining an area where a coal vein once existed, but has been picked clean by normal mining processes. It still gets product out, but it's a vastly inferior product.
The second link is a LOT more concise:
Which is, essentially, how I described it above. It takes all the parts that are left after every other process has claimed all normally edible parts, including many parts the average person wouldn't even consider eating, like brains, kidneys, and other vital organs, and via high pressure and high temperature, renders the rest into a paste.
The reason why MS Beef was banned in the 90's was because of the outbreak of Mad Cow disease and the concern that the BULK of MS meat is derived from the backbone (in other words, the bulk of the "tissue" derived therefrom was from the spinal cord) and the USDA did not deem the MS process to be sufficiently able to ensure non-transmission to humans, due to this high concentration of nervous system tissue.