Using the Rds(on) sensing isn't necessarily a bad idea; it automagically turns down the current if the MOSFET is heating up!
It isn't accurate, obviously, but you don't always need accurate charging current, just ensure power dissipation below maximum ratings. And Rds(on) sensing tracks that pretty well, if you have a transistor with higher Rds(on) or higher ambient temperature, it's doing the right thing for you. Careful with current sensing layout, of course. With a separate resistor, it's easier to do Kelvin sensing, but OTOH, separate resistor increases the loop area again.
ADP1850 doesn't have 90deg phase-shifted SYNC OUT - some ADI parts do have, I remember using one some time ago - so in order to have more phases than two, you would need an external clock generator. Any circuit which provides f_sw*2 with two outputs, 90 deg phase shift to each other would work. You could use them to drive two ADP1850's. ADP1850 internally adds 180 deg phase shift, so this would result in all four phases 0, 90, 180, 270 deg apart.
I would likely end up with 4 phases doing this, but well, do the calculation, including efficiency and price. My guesstimate is that with 4 phases, you can use the most popular commercial off-the-shelf parts for a simple BOM, but with 2 phases you may have trouble finding the inductors.