MDA is dead stop altitude with no margin.
DA can be breached provided you have initiated the go-around before you reach it (ie. you can sink below DA as long as the nose is up and power is applied)
HG
As for the altimeter error. As far as I am aware, that is accounted for when your DA is calculated.
It is basically aerodrome altitude + allowance (this is category dependent)
I think Cat C + D is about +150\' (but don\'t quote me)