[RPG] To what extent does Arcane Lock spell reinforce a door from being broken

Any normal means by which you'd open a door or window or break an object. It's just harder to do so (though just picking the lock may not be enough).

The last line of this spell handles this

While affected by this spell, the object is more difficult to break or force open; the DC to break it or pick any locks on it increases by 10. (PHB 215)

So if the DC to force open a locked window or door is normally 10, then one under the effect of Arcane Lock is now 20 (and so on and so forth).

Basically, the effect of this spell is to make an easy check a hard one, a medium check a very hard one, and a hard check nearly impossible.

As far as the text "impassible", it seems to mean in this context that the door/window can't be opened by normal means except with the higher DC. It's unclear to me whether or not picking a lock has any effect on this text, though it seems quite clear that forcing the door open is in bounds. I would definitely rule that at the very least picking the lock(s) on the door would ease the DC to force it open (As you're only dealing with the magic and not any physical locks).

Does this mean that a character making an attack against an Arcane Locked object bypasses the spell's protection, because the object's AC is unchanged? (Emphasis mine.)

RAW, yes, because making an attack specifically opposes AC, not DC.

The target number for an ability check or a saving throw is called a Difficulty Class (DC). The target number for an attack roll is called an Armor Class (AC). (PHB p.7, emphasis in original)

Arcane lock specifically changes "the DC to break it or pick it," not the object's Armor Class. (AC would be implied by an attempt to damage the object, rather than to break it).

That said, in the DMG's section on Objects (pp. 246-7) an object's Armor Class is described as "how difficult it is to deal damage to the object when striking it." So there's definitely leeway for a GM to read an object's AC as a sort of DC. But that's not RAW.