Avalonian miners use the coal to make hotter fires, and forge steel weapons. A great fleet of ships, both for escort and transport are assembling at Isla Vae.
Avalonia reveals the mission of the portion of its army that "disapeared". Their goal, which they have been very successful at, is to militarize as many "barbarians" into the Avalonian army as possible. Millions have joined and trained these many years, and been trained and armed superbly. Their families have entered into Avalonia's population, swelling it to five million strong, excluding combatants. These peoples have been moving to the Avalonian colonies in the south, which have been bulking up on boats, supplies, lodgings, etc.
It sounds unreasonable, but I've kept it secret for years, and so it is perfectly reasonable. By any of our timescales discussed, there has been sufficient time for this to happen.