There aren't any "openings" at all, the suit and helmet together are [i]sealed[/i] against the outside world (which is why the armor grants a resistance to radiation). The helmet locks into the armor, and the neck is protected. In fact, this Royal armour is actually something of an odd duck compared to the major armors over in America (meaning the T51-b and Enclave Advanced armors), as those lock in so seamlessly there's nothing but an impenetrable crease. And, stabbed in the neck =/= severed jugular or instant death, especially with the protection that I've just explained.

Look, all I'm trying to say here is, if Adrian attempts to use a knife to kill a fully armored Knight, he's not going to succeed. He's going to need better tactics than that.