I think this all boils down to what is the purpose of the flight from the outset. If it is a flying lesson (trial or otherwise), then it is aerial work. Passengers can be carried as observers to that lesson. If it is sold as solely a passenger flight, then it is flying for public transport. The selling of a trial flight with passengers and suggesting that all concerned can share the cost is clearly public transport.
So as I said at he beginning, consider what has been sold and the purpose of the flight. The purpose indicates whether it is aerial work or public transport. Whether passengers are subsequently carried or not does not alter the original purpose of the flight. Hope this helps.