FD's designed for 44t large rings (on 44/32/22 cranks) should work okay with 48t rings. And if they say max tooth 48t, then they'll work fine on 48, and will probably work fine on 50t as well. These sorts of things are usually calculated with some room for error.

I'm not sure if any road FD's are designed for top-routed cable.

But this isn't an area that I know much about. I looked over Shimano's website and couldn't find any mention of top-pull (or pull direction at all, leaving the default as bottom-pull) among any road-group front deraillers.