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.
Things are moving along with the duplex to single family home conversion.
The 20” floor trusses are ready to be cut. We strengthened them with 3/4 “ plywood, glued and screwed going back about 4 foot to the second vertical. We added our own 45 degree fillers and will be adding two horizontals at cut points.
We are using Post Shores to temporally support the trusses. Each post is rated at 5000 lbs. We’ve put up a handful to test. It’s working nicely.
Attached are a few pictures.
I’m going to add a few more posts for the “just in case” warm and fuzzy, I'm not sure how many posts are needed.