Would May, if she could be consulted somehow after death, approve of Newland renewing a relationship with Ellen in later life?
Considering that this is a hypothetical question, we could argue that May has not very much standing in several aspects of her life in Edith Wharton's The Age of Innocence. She is a woman made and molded by society, and not by life. She may have some say and opinion about Newland's life after she is gone, but surely her opinion will not involve her own feelings, but those of Newland: As an obedient and subservient woman, May's interests are uniquely concerning her husband.
May Newland's strongest characteristic is that she has the ease and capability of doing everything that is required of a fashionable young lady in 1870's New York. She does her part excellently obtaining, in return, the respect and admiration of those in society who are partial the people who know how to follow the rules of the house.
Ellen, on the contrary, is an ultra-bohemian and radically different woman whose charms make her even more dangerous in a prudish and snobby society. Yet, Ellen's strong characteristic is her loyalty to family. This loyalty for May and her respect for May's pregnancy are the factors that make her walk away from Newland, causing immense sadness to Ellen.
All this being said, May probably would not criticize Newland for finding love again should she die. She probably would not criticize him even if that other woman is Ellen, because May lives her life only to look for Newland's best interests, first and foremost. However, accepting Ellen as a companion for Newland would not come out of May's giving heart, but out of her dutiful loyalty to Newland, whom she places in a pedestal much higher than her own.