Please quote where you have seen me claim a 12V lead-acid battery managing 15V during discharge. A full cell have somewhere between 2.05 and 2.1V - not 2.5V. 10V is a voltage you should avoid to go below expect for short high-current discharges, since too deep discharges destroys or quickly ages the battery. 10.5V is a better limit unless having a battery specifically designed to sustain many deep discharges, our you just don't care about the ageing and potential instant failures. And 15V is the highest voltage you should apply externally when charging it, since you don't want the battery to start to boil. You should not go higher unless in cold weather or pulsing the charge current.

The only important thing about the charge voltage is that it must not destroy any connected charge meter. But it is totally irrelevant to the scale of a voltage-based capacity meter. With the main exception of smart current-integrating chips, most battery capacity meters (NiMH, lead-acid, LiIon, ...) will not be able to predict anything while the battery is being charged - the applied charge voltage is way outside the possible voltage range from the battery cells. Many cellular phones settles for animating the battery bar until you disconnect the charger. Since the charge voltage is so much above the discharge current, nothing can be said about the charge stated by looking at the charge voltage.

The current for the LEDref is constant in the context it is used. Even if you throw all your money at the problem, you will still not manage to design a circuit with absolute perfect line regulation. Whatevery circuit we design will just be more or less good at line regulation. In a practical world we can never get true "constant" - we can just get "good enough". If one LED have the current varying one hundredth of the other LED, the LED current is practically constant for that application. It's the same thing with your Allegro chips. They are not even close to fulfilling the mathematical term "constant" but for their intended use, they can be considered constant-current.