Well, it sure can be tricky to judge cooling system performance on these engines for sure. Just a few comments, first using an external water source connected to the side bypass cooling cover doesn’t provide too much information. The pressurized water acts as the water pump, so a bad/worn water pump can not be judged using this external pressurized cooling water source. Next, I am concerned about how deep your gearcase is immersed when you run the engine in a bucket, I don’t understand your written description. The gearcase must be immersed so that the connection line between the gearcase and exhaust hsg/extension hsg is under water. You mention that water pump pieces were replaced. It is extremely important that there is a good seal between the SS impeller plate and the gearcase hsg surfaces. These areas are rather thin, so erosion/compromised casting areas will create a possible air leak under the pump. Were all the threads for the water pump screws in good shape? No oversized screws or other shoddy repairs? Usually, these issues only occur on salt water engines, but they can occur on fresh water engines also. I am assuming you used new OEM water pump pieces.
You are so right about pictures/videos being worth a thousand words, especially my words. So, post a video of the engine running in a barrel/bucket for us if possible.
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