To take into account the present day situation on security based platforms, "rogue states" have far too much to be worrying about to be thinking of us right now, and in fifty years or so when we actually have something to defend it will be out of their reach. Any "space strike" capability is easily mitigated by a variety of means. The distances involved will allow more reaction time too. We should be able to neutralise anything as or shortly after it leaves atmosphere.
"Terrorism" isn't much of a concern as I doubt MI6/CIA (and other firms that sponsor, supply and direct "freedom fighters" in order to destabilise reigons) will be able to get them clearance to board anything that can get close enough, and should have nothing with which they are able to corrupt our citizens with. Religous extremists who feel that strongly will be far too afraid to leave the surface, in devil technology.
Nothing on Earth should require our intervention, it's simply outside of our remit. Who are we to define ourselves as an international police force, invade other countries and attempt to impose our will upon them? To ignore the contradiction there in the founding ethics, that really doesn't sound like a clever game to start playing. If we're going to do that, we might as well rebrand to "America" now. Sure, we should attempt diplomatic means in order to facilitate resolve, but attacking anywhere isn't an option. There's already setups for such goals anyway, for example the UN's peacekeeping forces.
A defence system is indeed required, but thankully this is actually one of the easiest possible problems to solve, having already been solved decades ago - it will be harder to get UN recognition as a valid nation, even selecting a flag or an anthem represents more of a problem.
The best way to contribute to the global fight on terrorism is to directly pull down the infrastructure that spawns it. Unfortunately, due to the aforementioned demiliterised and peaceful use of space limitations, physically taking them out isn't an option so that leaves diplomatic means again - and after investing so heavily in their "intelligence agencies" I don't see countries trying to get rid of them anytime soon - especially as all their neighbours have them and are using them in an offensive capacity also.
It's odd you mention a missile defence system, because if you'd done any research you'd know that's definitely covered - when it actually becomes applicable.
As to it's structure, we'll certainly require transport but "aerial units" isn't quite the right terminology, as these rightly should never be entering atmosphere. Ground forces are indeed useless, as we have no - and will create no - plans to invade anywhere. For the basically laughable concept of lifting troops for an invasion of our facilities, where they will then be vastly outnumbered and drastically under-equipped, to assume we actually allow them to make the journey to us by not interfering with things like their propulsion or guidance system it will leave them incredibly vulnerable if we decide to use lethal means to prevent.
An air based flying military would be entirely useless. For a start, it's not like a space ship handles like a jet. It's very different. Combat can be reduced to simply taking out their radiators and letting them cook themselves or heating their fuel tank(s) enough to ignite. Besides, which part of "The third goal is to create a demilitarized and free scientific base of knowledge in space" or "First, is to ensure the peaceful use of space" did you find particularly unclear or difficult to understand?