No, do not change the claim.
Firstly this has time wasting nonsense by the defendant written all over it. Secondly she is a joint owner and thirdly the state of his marriage is none of your concern. If you get the judgment you are free to pursue it solely against one or both defendants meaning you can pursue it solely against him and let him apply for any contribution he can get from the wife.
In short not your problem
I hope this helps if there are any further points please reply.
This has been going on for over two years now durning which time I've tried to find a compromise of delayed or even monthly payments until finally being told they would't pay.
Although I see his wife on a regular basis, in truth I couldn't prove she lived there and I thought, as you do, that there was some 'method in their madness'.
I will take your advice and leave things as they are.