Well it is impressive background, and much better than I could do!
A couple of things though... to me it seems that the creation of the Scions of Iron chapter which emphasizes vehicles and armour, would make more sense as being an even MORE extreme version of the doctrine of the Iron Hands - ie the more machines (vehicles) the better. So I don't see that the 'hey maybe chopping off our hands thing isn't necessary' clause fits with that.
The other thing is that according to the background, the Iron Hand Clans are already practically chapters in themselves (the Iron Hands don't have companies like other chapters, just clans which are basically independent of each other). Thus just one Clan taking the doctrine to zealous extremes, and growing larger, would be enough to form a new chapter. I would prefer a gradual process of alienation and isolation from other clans driven by increasing zealotry rather than a breakaway plot by a few marines, which seems harder to justify IMHO.