HJ-
The inner marker certainly does give a decision height. If you are established on a 3 degree glide slope, when you cross the inner marker, you are at 100 ft.
We are not permitted to use a barometric altimeter to establish DH on a CAT II approach. I checked with 3 Air Carrier Inspectors from the DTW FSDO and they don't know of any carrier that is. (That doesn't mean there aren't, just that they don't know of any.)
In any event, at my airline, if the CAT II approach shows "RA not authorized", our DH is crossing the inner marker.
At your airline, if the Radio Altimeter is inoperative, are you permitted to shoot a CAT II? If so, do you use a barometric altimeter to determine DH?
(I can only speak to the rules in the US, other states may be different.)