The Agreement with my wife after she dies

Formaldehyde is a highly toxic systemic poison that is absorbed well by inhalation. The initial reaction is eye, nose and lung irritation and bronchial restriction that causes pain, coughing and breathlessness. If the exposure is high enough, this is followed within a few hours by the lungs filling with fluid. Even if the respiratory crisis is survived, the formaldehyde is metabolised into formic acid in the body, and this causes an acid-base imbalance that can lead to cardiac problems, renal failure and other organs being damaged.

So the first time you tried this would almost certainly also be the last.

You could wear a breathing apparatus, but formaldehyde causes chemical burns on exposed skin. So unless you're into serious pain, you'd probably get just as much pleasure out of the experience as your deceased wife would.