The spell does say what happens.
...if the object is moved more than 10 feet from where you cast this spell, the glyph is broken, and the spell ends without being triggered...
I would apply this to breaking the object in half or scratching the glyph. As for it it being too easy to disable, keep in mind the glyph is nearly invisible. You're not going to spot it just walking past. You have to be looking for it, or at least investigating the area. It's not going to be easily found.
There's also the idea that you can set one of the triggers to be if the glyph is tampered with so it does go off if someone tries to disable the glyph.
The creature that triggers the glyph will be teleported to a place the caster chose upon casting
Your second option is the correct one.
Misty step has a range of self, which means it only targets a single creature. As such it is qualified to be cast and put into a spell glyph.
As for what happens after it triggers, glyph of warding says:
You can store a prepared spell of 3rd level or lower in the glyph by casting it as part of creating the glyph.
You cast the spell fully when you put the spell in the glyph which means you also need to choose the parameters of the spell, just as if you were casting it normally.1
When the glyph is triggered, the stored spell is cast. If the spell has a target, it targets the creature that triggered the glyph.
Note that the creature who triggers the glyph is not considered to be the caster of the spell, it is considered to be the target of the spell. Essentially, the glyph casts the spell by proxy. Normally this means that the creature would have no input on how the spell works it would just receive its effects. The ambiguity here is that misty step has a range of self which means that normally, the only person who can be targeted by the spell is the caster.
In this case, however, glyph of warding overrides that and makes them the target of the spell even though they are not the caster. And, because the spell has already been fully cast, there is nothing else for them to choose.
Thus, they are teleported according to the conditions set on the original casting of the spell/glyph.
1 - This means you also technically would choose a target/area for spells in general when you cast them. However, after triggering, this is the one parameter that glyph of warding overrides.
Best Answer
A Glyph wards an area or an object. While the trigger can be refined to be conditionally dependent on (almost) anything you want it still probably necessarily needs to interact with the area or object you've warded:
The above quote, preceeding the section on refining the trigger, does not indicate that the trigger can be anything but rather that it can be lots of things and also lists several things it definitely can be. If you go outside that list for the basic trigger, you need to confirm with your GM that the new trigger is possible. Unlike the open-ended section on refining triggers, it is not the case that the basic trigger can accomplish what you want.
Furthermore, there is historical precedence for it not doing so; Glyph of Warding in earlier editions of the game, while always very open ended, specified some version of the following clause (taken from AD&D 2.0):
which indicates the general scope of potential triggers-- they must in some way relate to the warded object or area. Refinement, however, is not nearly so limited.
This means the following death triggers are possible:
But the following triggers probably don't work:
In conclusion it's not proximity but interaction that matters, but proximity is usually a prerequisite of interaction. Non-local parts to trigger's refinement are fine, but the trigger itself (very probably) has to involve the warded object or area in some way.
Addendum for the specific purpose of dead-man-switching a WMD:
You can do this with the spell, and it is very appropriate. What you want is a glyph on each weapon that triggers "When a person not bearing the Royal Seal attempts to move, activate, or dispel the object, or if the object is moved, activated, or subjected to dispelling forces in any way and I am dead", and then you just tell the kingdom about the first part. You can't make the devices trigger upon your death but you can make them (and any other devices you feel so inclined to affect) work fine until you die and then turn upon their masters if they are ever used past that point. Basically, you are creating a cursed item whose curse's activation is delayed until your death.
This kind of warding, though, is still somewhat risky, since the glyph can be dispelled if the dispelling is done carefully, and then replaced with you none the wiser.
You could also ward yourself with a (surface, and thus mobile) Glyph that triggers when someone approaches or strikes the surface with the intent to kill you, and have that Glyph somehow remotely trigger the deadman operation via your choice of long-range communication spells and recievers, but you should be aware such a measure can be triggered against your wishes by a clever opponent of your home nation, or bypassed via clever assassination methods (e.g. poison for the specific wording I mentioned)
Basically, this does almost exactly what you want it to for your linked answer, but is a lot less infallible than you might have first thought.