"I have often assumed, and we all know what means, that the Taurians were also building Trojan at those Union lines, or the hidden line for them, owned by the Far Lookers."
But you won't ever see them retcon a book to reflect that. The Taurians making Leopard's, Union's, Union conventional or ASF carriers, Trojan's and all its varient's is never updated in a later book. Plus, every major(TC, MoC, OA) nation should have the ability to make a few jump ships each year. That is how the "Universe" keeps them from being major players.
The Marian's have the best chance to build from the ground up. Lots of techs, companies and colonists hoping to flee the inner sphere to a "better life". Lots of Germanium and in the neighboring planets, resources. Plus, what company getting pummeled during the succession wars wouldn't want a secure factory that could be built to their specifications and well protected.