I’m not sure if its really an issue of ethics during DD. To some sense I don’t think investigating parties would ethically violate doing a proper DD. Sellers would of course try to ‘manage’ a DD to ensure the deal for their shareholders, but even then I believe that with all the legal clauses in place and the fact that many sellers will end up working for the buyers, there is some degree of check and balance built into the process.
There are also other mitigating factors such as time and resource constraints that prevent full DD being completed. Thus it is always a matter of identifying the key risks and being able to address those. Of course any deviation from base assumptions that materially affect a deal need to be highlighted, but that’s more a company culture thing than an ethics issue IMHO. Also, there needs to be a sense of accountability in place for those conducting the DD and those approving the deals.