It is a challenge at the best of times to achieve a legally enforceable spousal support separation agreement with a former partner/spouse. Is it legally possible to achieve an enforceable "air tight deal" when the former partner/spouse is mentally ill? We say: "unfortunately - no".
The Supreme Court of Canada decision in Miglin applied to support cases under the Divorce Act. Thrust of Miglin? - If you make too good of a spousal support deal, if you go beyond the broad parameters of the principles inherent within the philosophy of the Divorce Act, then a later court might over turn your Separation Agreement.
The Supreme Court of Canada case - Rick v. Brandsema - says that the Miglin analysis applies to the common law as well. In other words, even if you are not formally married and you make an agreement that applies the family law of Ontario, then the Miglin analysis still applies. If you make too good of deal whether under the Divorce Act or not, then a later court might set it aside.
Certainly, a failure to make financial disclosure or making inaccurate disclosure will constitute grounds to set aside any type of Separation Agreement or Marriage Contract. Miglin and Rick v. Brandsema go even beyond that principle. Even where your financial disclosure is perfect in terms of completeness and accuracy, if you achieve a result that is beyond the pale in terms of reasonableness under the applicable legislation and case law under that legislation, then you stand at the mercy of the court should your former partner/spouse have a change of heart years down the line.
In Rick v. Brandsema the wife apparently had mental health issues that the husband knew about. Even though the wife had two successive lawyers, professional accounting advice and had participated in two mediations, the SCC nonetheless would not permit the deal to stand.
We conclude from Rick v. Brandsema that it is not possible in Ontario (and in Canada) to make an enforceable separation agreement with a wife who has mental health issues that are known to the husband. If such an agreement is made, the husband must be prepared to face litigation later should the wife decide to return to court. Where there are mental health issues plus some element of nondisclosure or inaccurate disclosure, then our concern for the agreement's enforceability is magnified.
Financial disclosure must be pristine in its accuracy and completeness. Anything less exposes the husband (or ex-husband) to further litigation down the road.
The bottom line implication for family law practice is that Separation Agreements with mentally challenged spouses are to be avoided. It is preferable to formalize the financial arrangements within a court order (on the consent of both sides who are independently represented) where a judge signs that order.