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HI and thank you for your question.
This certainly can be a frustrating experience.
Due to the chronology of the events you shared, I would suggest pulling the pump out and getting a better look at the impeller. I suspect you picked up a rock and bent a blade. The rock went through the pump, did its damage and is now long gone.
While you have it out, you can also check the rear bearings separate from the front support.
If you have any more questions, feel free to ask.
Yes and no, the answer to your questions in the order you asked. However, it is highly improbable that it has come loose. It is designed with reverse threads that would encourage tightening, not loosening.
I understand you ran it out of the water with no symptoms, but it is still not operable in the water. When the pump is not in the water, there is no resistance, so to speak, to the pump turning, and therefore not a great test of pump condition. We know something is wrong, we just don't know what, yet. Therefore, we need to be able to scrutinize the assembly, and the only way to do that is to remove the pump assembly. It is exactly what would be done if you took it to a shop, minus the several hundred dollar service bill to do so. You wouldn't want a shop to leave any stones un-turned, and neither should we. If they gave it back to you saying it isn't making noise on the flush kit, would you feel like they had helped you at all?
Let me know,