And yet you still can't prove that it wouldn't have an adverse effect on Mercer, he's a walking virus, what exactly is to say that if he tried to take Dante that he's even capable of taking demonic DNA without consequence, if at all. The Mercer Virus wasn't designed with demonic creatures in mind and without that information you can't say what would happen.
I never said it wouldn't. But considering he can take on DNA of any animal, it could be a possibility that he could consume demons. Plus there's still Dante's DNA that's part demon but also part human as well. He could still consume Dante, and who knows, the DNA doesn't even have to be human for him to consume, so maybe a demon or in this case "magic DNA" won't have an affect on him.
Putting that aside for a moment though, you have to get into physical contact for that to happen, Dante can just take him out long range simply by DT bullet helling him, i severely doubt that even with Mercer's enhanced defences that he could just shrug off attacks that could take down a being like Mundus.