ANY movement of the toilet will break a wax seal...it isn't rubber, once compressed, it stays there. So, if you want to stay with wax, you must stabilize first the floor, then the toilet on the floor. The CI flange should be anchored to the floor so that the relative motion between the floor and it are stopped. Now, if the floor is weak, you really do need to shore that up.
The waxless seals rely on a soft, large closed cell rubber o-ring for the seal. For these to work, the insides of the pipe must be fairly smooth and clean to start off. On a really old CI pipe, that may not exist, so it won't seal well. There is one company mentioned in previous threads that is making a foam rubber waxless ring that is used in place of a wax ring. That might work for you.
So, first, shore up the floor so it is strong enough to not deflect. Then, if the inside of the pipe is fairly clean, you could try a waxless seal (Fluidmaster and Fernco are two I'm aware of, not counting the new foam ring). If you try to go with wax, then you'd probably want to either fix the location of the flange - it should be on top of the finished floor, or less desireable, use flange extenders on it to raise it. A double wax ring can work, but it can get risky if you ever need to plunge the toilet...it could create a blowout situation if you get really active with the plunger.