One saving grace with a twin is that you swap items from side to side as a troubleshooting exercise that makes life pretty easy to see if the fault goes with the component.
A couple of things that you may have missed that you didn't list but no doubt have looked at -
Anti-ice valve good? From memory (which may have faded as it's ~30 years since I worked on one) they are spring loaded to one direction cant remember which. Cap both lines and do a ground power check.
Bleed air heater/demister leak - from memory the lines are alloy and water accumulates in the low point from compressor washing and the line rots from the inside out before the valve. It is not obvious as the line has a heat sleeve on it and you cant see the holes in it. 350 has the same problem. Fix is to run the heater regularly after each wash to vent the line and dry it. Again cap all the lines right at the engine and do a ground power check.
Your TQ split is a pain on the 355 and will depend on your governor condition. From memory again I think you can adjust the bias on the droop compensation.
If you are really in a bind swapping engines from side to side isn't that difficult.
Talking to Airbus on a model this old would be about as effective as asking your grandmother unless the tech rep is really old.