Floor Truss Shortening
Any of us that have built hotels, motels, military housing, etc... have likely done something like you originally asked about, which is cut some trusses to a shorter length for one reason or another. You have no contractor, so you are the go between for the engineer and the truss manufacturer, as someone else stated. The truss manufacturer's engineer would be my choice for advice on strengthening the cut member. Sometimes they want to come press on a gusset plate, sometimes they recommend you glue and nail plywood. My preference in lieu of a factory installed gusset is the plywood gusset. Whatever you come up with, doing it yourself isn't very difficult to do if you're motivated and reasonably intelligent, as you clearly are.