with a very simple fix. Since it works when you hold the throttle lack of friction is the problem. Don't overthink this into a major, expensive issue. As others have said, there is not enough friction in the cable to counteract the return spring. Morse makes a clamp to add friction by pinching the cable. When I needed one I couldn't get one easily so I improvised. There is a kind of wire clamp for joining large wires. It is a copper bolt with a slot cut through it to hold wire. The nut has an insert in it to clamp down on the wire. They are easier to come by than the Morse fitting. Just tighten to the point where the throttle holds still.

PS. I see you already saw everyone's opinions on coarse vs fine threads.

PPS. Glad you had a good trip. It has been a rare passage when I didn't have something to fix on arrival. Enjoy the Azores.