"Can conlonize each other systems" means you can ONLY colonize in the other's systems if that's turned on. You can have a full military treaty and still not allow that.
Who gets what is determined by who plants a colony there first, as far as I can tell.
The position of components does matter. Once your shields and armor fail, damage is directional- if the hit comes from behind, components at the back will die first.
Speed is life if you have short range weapons, yes. That's fairly basic tactics.
What a human player could do, and the AI sadly won't, is split his force to try and hem you in from multiple directions. At least in SE5 you can't do the "missile dance" anymore.