OK, I give it a try. But lets consider the possibility I like it and want to send a big amount later. How can we prevent the situation that I send you a large amount of money and someone else just claims it is his investment, resulting in two different return addresses. How would you decide who is lying?
You should be sending the PM immediately after depositing into the fund address. I'd say have the message typed out already, with just the TXID ready to be pasted in. It has never come down to a situation where there was a dispute between users over a deposit. If it ever does come to it though, additional measures would be looked into to prove address ownership.