The Tierran navy would be more useful against Kian then Takara as Kian like Tierra lacks the ability to sail against the wind. We’d probably have to either disperse the fleets or keep them within range of major fortresses such as Leoniscourt, but at least we’d be able to force Kian to sail in large armadas if they didn’t want to get picked off.
Keeping Leoniscourt and the Northern Pillars out of Takara’s hands would be an even tougher challenge. They need to be given the absolute latest arms, especially artillery, that Garing can make, and stocked with several years worth of food and ammunition. Fresh water too if it isn’t feasible to distill or set up distillation of sufficient quantities. Smuggling in supplies will become much more difficult if these two island fortresses fall.
Not more likely. This will be part of the invader’s plan, but they will attempt to wrap it up and turn it into a fait accompli before their rival has time to react. Miguel will likely be assassinated and replaced by a puppet who controls the capital and is armed and funded by the invading power. This will launch a rebellion which in turn will be supported by the other great power.