Regardless of what the outcome will be this should serve as an important lesson for everyone to never deposit before making sure you saved your Note Private Key. If you have access to it you won't encounter any issues.
Looking forward to your opinions.
You are completely independent of all the decisions you have to make in solving customer problems. Every customer that comes claiming to have deposited a certain amount must be proven by valid evidence, if they can't do it, then what are you worried about?
I don't understand and probably disagree that some other people should be held responsible for other people's mistakes, so I don't think it's a good decision for you to consider in that customer problem solving process. Regardless of whether you have good intentions for every customer who complains, but I think every customer should and must comply with all the rules and how your service works.