Can they atleast show some proof that they have indeed received the 400 ETH they claim to have received privately through investors or angels
Yes, in my opinion, havd transparency is better for this case. If have address tracking will make people have belief about Fixy network more. I have no doubt about this amount, but i am really curious about this.
Practically in all projects the key address of fund raising is always hidden and nobody knows exact numbers, so also this project not the exception was led. And in general, often the unique new generated address is appropriated to each investor.