Re: Mirrors of Opposition:
I found it hard to swallow that a 90k gp magic item(s) could be destroyed by a rock, so I arbitrarily increased the hardness and hit points.
My rationale: I treated it like magic armor/weapons, wherein each +1 of enhancement bonus increases hardness by 2 and hit points by 10. The CL of the mirror is 15; magic armor made by a CL 15 caster can have up to a +5 enhancement (+1 per 3 levels), so my mirrors have hardness 11 and hp 51 (assuming 1" thickness). That's just my reasoning, your mileage may vary, of course.
Using the rules for energy attacks under Damaging Objects, I speculated glass would take half damage from acid, cold, electric, piercing, and slashing damage; and full damage from sonic or bludgeoning (although the rules for damaging objects further suggest double damage from a specific vulnerability, or bypassing hardness).
As an aside, I think it would be kind of cool to describe to players how a simulacrum generated after the mirror is damaged by acid or fire (for example) might appear to have pustules on its face, or charred bits of flesh.
As another aside, it just occurred to me that following up cold or fire damage with its opposite, or any similarly significant sudden temperature variance, should probably overcome the half-damage and/or hardness rule...
My challenge is that my group is 6 PCs and 2 cohorts with a total APL of 16.5-ish, having JUST entered Runeforge when the chapter advancement track suggests they should be APL 14. Sooo, I have a lot of "adjustment" to do in terms of encounter challenges :)