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c) Don't care about the instructions and simply work out the technically best solution. My only task is to give professional advice

This is the conceivable worst solution for both sides. The customer has the feeling that you don't respect him, work past him and deceive him. Sooner or later this will lead to a clash. There is nothing but to talk about the problem. As a matter of fact, this is something that Chinese usually try to avoid at any costs. Therefore talking about a problem must be done with caution and should be expressed rather indirectly.