Unfortunately, the ADP8866 only has one I2C address. You could take a look at the ADP8861 or ADP8870, and see if they would work for you. The programming and feature set for them is different though.
No, the minimum is 15mA. For lower current, I would suggest the ADD5203, ADP8870 (or ADP8866), or the ADP8140. The ADP8140 has the best DC dimming and DC dimming range.
No, the ADD5211 or ADP8140 cannot control each string individually. However, the ADP8140 can be easily connected in parallel, so that you can control up to 2A of LED current individually on each ADP8140. But it requires a separate IC…