I kind of wonder if your tiny 12v solar panel would overcharge your 6v battery, even with a direct connection. The panel is essentially a constant current source and its voltage is going to track that of the battery. Even so, just to be safe, you could hang a 7.5v 5W zener off of the panel (where D1 is at in your schematic) which would serve to keep the voltage within the limit of the 6v battery. Make sure you keep D2 in series on the battery side of this junction to prevent the battery backfeeding into the solar panel (at night) or the zener. You could then eliminate U1, etc.