There are numerous unknowns in the mysterious death of Ms. Romain. Still, the facts appear to make suicide unlikely.
No motive for suicide was suggested by the facts. It makes no sense that a devout Catholic goes across the street from an evening Mass to jump in icy water. Her new purse was damaged, suggesting a struggle at the car. It is easy to make a copy of a key. Still, shallow water far from the channel makes it unlikely she would not have been found that evening had she entered the water across from the church.
The red flag is her brother's debts. How much did he owe and to whom? Who inherited what in the aftermath of Ms. Romain's death? Even if the brother was not directly involved, dangerous people may well have benefitted from the distribution of the estate. Why did the son choose to remain silent? What did he inherit and did he go in on any real estate deals with his Uncle? Did the estranged husband stand to gain financially from the timing of the death? Why did Ms. Romain feel the need to contact a private investigator? Was she being followed in the days/weeks leading up to her possible abduction? Chloroform and a ride to Detroit in her own car seems quite possible. Suicide by drowning in icy water as a way to go is far fetched. No note. No talk of despair. No change in behavior other then the belief she may be under observation. She kept to her routines, which would have given pros enough information to hatch a plan. The PI raised important doubts and questions, even if abduction could not be determined after the fact.
The daughters had plenty of time to recover from the shock and to consider all of the possibilities. Neither has waivered on their belief that the mother was in no way suicidal. The police jumped to a conclusion and the case was quickly closed. It is not conspiratorial to take the path of least resistance and move on to the next case. This reeks of a professional job. Condolences to the daughters.