Hi Belle
There's a difference between saying the DWP
would have a legal case to prosecute you (which I think is what we've already said here, and also what your lawyer is saying), and that they
actually would prosecute you, which would be a decision taken on a more individual basis.
If you have this money in a bank account or wherever that you haven't declared, and they are really determined to find it, then they can and will find it, whether you admit to it or not, so I don't think it's correct to suggest that the only way they could prosecute is if you incriminate yourself, though I suppose that would make it easier for them.
On the other hand, if they say at your interview that they have been informed you have money you haven't told them about (because it is still possible that this whole thing is about something else) and you come clean, explain why you didn't let them know and offer to pay back whatever you owe them, then they might be happy with that.
I would suggest you need to try and find out somehow
how likely it is they will prosecute, and the only way I can see of doing that is through CRB or someone similar who have experience, hopefully, of helping people in a similar situation to you. Unless your lawyer has experience of a number of similar cases, I'm not sure they would be able to judge this question very easily.
I hope that makes sense, and is of some help