I understand from your post that you tossed the ribs. If you didn't toss them, here are a few thoughts.
The first concern would be food safety. I'd be worried about how long that slabs were in the temperature "danger zone" (between 40 and 140 F). At what temperature were you ccoking the ribs? Did you have a thermometer probe in them? In reading you post, it sounds like you are grilling the ribs, rather than slow smoking. Do you have any idea what the cooker temp was, or what temp the ribs were at when you pulled them?
With all that said, you're probably still ok, food safety wise. I read that you pulled the ribs, wrapped in fresh foil, and placed them in the fridge. If its just one slab, they should have cooled to the proper temp in no time.
Now, my ribs of choice are spares, which are generally larger and meatier than baby backs, so take my suggestions accordingly. Your post said they were "hard as a rock". If you normally have good results cooking babybacks, it says that either your temp was too high, and they got burned, or if your temp was right, and you just didn't cook them long enough. Tough cuts of meat (like ribs) become tender by the fat rendering out of them over a long peroid of time. There are also other ways to make these cuts tender, but since were talking BBQ, we'll leave the rest out.
So, if you've properly cooled your ribs, now comes the time to re-heat them, and finish cooking. You said you re-heated them in an oven. You can do this, but do it gently. I recommend 225F, and 250F at the most, only if you must. As for the foil, I'd be inclined to leave it on. Your ribs will have less bark, but they won't dry out. This sort of comes back to your cooker though. Remember that these ribs were not cooked fully to begin with, so give them time at a low temp to tenderize.
If you cook them this way, be careful of overcooking. Leaving the foil on will result in a steaming effect. If you overcook them, you will have "falling off the bone ribs". To many, this sounds good, but you will never be able to cut a slab like this into individual ribs. What you want is an individual rib that when you take a bite, the meat come away cleanly, without effort, but the meat is not just falling off.
Use a probe thermometer to chack the temp of the ribs before pulling them. You want at least 165F for food safety. As for the tenderness, grab one of the bones. Shake it a little, it should move independently from the slab. It won't be quite as pretty, but if your not sure, pick a bit of meat from the meaty side of the slab. A string of meat should come of easily and the meat under should be moist.
As a final note, please take these suggestions knowing that I'm a sparerib guy. I've cooked babybacks, and have had good results, but I maybe cook 30 pounds of babybacks a year versus 40,000 pounds a year of spares. I'm thinking the principles are about the same, it's just that spares are easier to do well on a consistent basis.