Oops my thinking was a bit backward regarding the military construction rules.
The platform should be considered a military building if it hosts any military building. Failing to do so enables the HQ to become indestructible, guarded by a civilian platform that cannot be destroyed as long as a military building exists.
When the platform is considered a military building, no rule prevents it from being destroyed and taking an HQ building with it. Other military buildings that were not on the platform could then exist without an HQ. They could not have been built that way but they should not pose a problem.
The platform should be considered a military building if it hosts any military building. Failing to do so enables the HQ to become indestructible, guarded by a civilian platform that cannot be destroyed as long as a military building exists.
When the platform is considered a military building, no rule prevents it from being destroyed and taking an HQ building with it. Other military buildings that were not on the platform could then exist without an HQ. They could not have been built that way but they should not pose a problem.