2) if you have made a boolean difference "surface" was be automatic ! No need to close curves etc... :)
Never "Trim" a volume if you want surface! Always use the Boolean Diff between a volume and a curve/surface/curve !

3) now you can can fillet the whole piece!
but the open surface will be not filleted!

you must close it before
alas there not yet close function for close a general hole!
You must make something like this : redraw a middle curve curvated, trim the edges with this new curve, Network etc...
so the boolean Diff was indispensable for avoid all that !!! :)