Zoners didnt originally develop their own craft, they used civilian and borderworlds lines. They import a lot but only have one major export - deuterium. By the terms you put up here, bacon, that would mean zoners are not their own 'sovereign house' - yet they hold more territory than most houses, have their own government (council), and are rich... filthy rich... probably on par with house governments.
You do not have to manufacture your own craft to be sovereign. Look at other countries in the modern world, there are many 'sovereign' (i.e. self governmented) countries that import nearly everything they use. If you have one major export, you can import everything else you need with the money you make from that export.
The GMG is, for the most part, the sole provider of H-fuel to all four houses. Samura is -trying- to develop it's own gas mining ops in hokkaido but has been unsuccessful. The GMG dont ship into rheinland themselves, but they sell the fuel to kishiro which does ship it into rheinland. Basically, the GMG have one of the most valuable exports in sirius, which gives them more than enough money to outsource ship construction and anything else they need made.
So... I disagree. I think that the GMG is it's own sovereign house, though it's very small compared to some of the other houses - just like the Outcasts, the Corsairs, and the Zoners. I would even go as far as to extend that to include the IMG.