Lots of questions, doubt if I can answer all of them.
Forsythia, cut some old stems to ground level after flowering. But if they're breaking anyway you might as well take them off before the winter gales..
Apples, branches that are still bare are dead, cut them back to something live without leaving any long stumps that will die back.
Buddlejas respond well to cutting back in spring.
If you cut back the willow, drill and fill with Rootout (or similar) and cover with a black plastic bag, tied on, it should work and won't poison anything else.
I cut hydrangea arborescens back hard to a well placed bud in spring, (or I did before it died of drought last year).
I hope someone else can help you with the rest.