Instead of using the built in window sill option, use a trim profile and select to change to unique trim for each part. Then you can have a stool and apron or sill and casing under your window.
For the fascia extension, my guess is you will have the least trouble if you just draw a profile to match your fascia (while in 3D) and extend it out where you want it, but you'd have to do that with the gutter as a profile as well.
Another option is draw two roofs. Adjust gable overhangs or whatever the neighboring roof overhang is out to where you want the extended gutter and fascia to be, then hide all but the fascia and gutter you want shown extended.
Or, make a profile assembly with gutter and fascia all together and draw it as a trim item (I like chair rail because I rarely use it for anything else and it historically has been easy to adjust) while in interior mode, but adjust and place it so it will be in the correct plane as your fascia on the roof eave you want extended.
You could also make a symbol of such an assembly and just offset it to the correct position needed.
Lots of ways to do it, except the easy way of having an edit menu on the roof menu and being able to extend fascia, gutter, roofing or whatever right in the edit edge menu. Such control sure would be nice.