FD has a history (with JD at least, don't know about his relationship with his previous wife or why they divorced) of being intimidating, threatening, and controlling of his intimate partner as documented in court proceedings by his wife of 15 plus years. JMO, I see no reason why he would suddenly stop being that kind of guy with his next partner, MT, regardless of love or convenience or whatever brought them together.
Whether or not people think MT's in the wrong for being with him prior to and during the divorce proceedings, it was also his choice to cheat on and then leave his wife and move in with MT and her daughter. He's the most morally bankrupt of the pair of them IMO for breaking his wedding vows and leaving his family to be with his 'paramour' and borrowing millions of dollars from his wife's wealthy family he has no intention of paying back. I bet JD was relieved he was with another woman so she could distance herself from FD bit by bit cuz he was soooo scary, she told the courts she was afraid of him and he had threatened her with violence, absconding with their children, and spelled out revenge fantasies to her about what he would do to people who had wronged him.
Therefore, I believe it's possible that MT could be scared of him or intimidated by him or is an emotionally or otherwise battered domestic partner IMO, even if it was just a dawning realization over the last few years she's been with him. It's classic behavior IMO for someone who may love their domineering partner but lives in fear of retaliation or worse if they don't go along with things, sometimes doing things they wouldn't otherwise on their own to keep the peace with someone who initially seemed charming and won them over, but later revealed themselves to be quite off (to say the least).
It could even be as simple as he was a 'successful businessman' she had worked closely with, and he was charming and he told her he was leaving his wife, and she fell for him, and/or she thought he was a good provider for her and her daughter who settled there as citizens from another country, and she towed the line and went along with whatever he said to keep the peace and maintain stability and the nice big home for her and her daughter. No one knows what he told her to convince her to go with him to Hartford to drop off the bags with JD's blood in them. No one knows what goes on behind closed doors with a couple, but IMO MT looks pretty haggard in court photos / is not a happy camper. She could have been secretly miserable sharing her life with him of late, you never know. I just don't buy that she's the bad actor or even an equal partner in all this (JD's assault & disappearance), or was even complicit in JD's disappearance if she was under duress to help him after the fact at his insistence with threats or innuendos about what would happen to her and her daughter if she didn't go along with his directives.
This is all JMO, but I just don't give FD an ounce of credit for being a standup guy towards anyone he knows, and think he's a very disturbed, manipulative psycho/sociopath with a track record of bullying & intimidating & possibly worse (the 'freak accidents' in his family) and there's no reason in my mind he would have treated MT any differently (once he won her over and had her in his clutches). Time will tell if she has acted under her own free will as the investigation proceeds, or there was something much more sinister going on in their relationship. I can only hope that she gives LE some exclusive info that helps them find JD as quickly as possible. I'm encouraged that she has separate legal counsel, they are not in contact with FD or his legal counsel, and she met with detectives twice last week for several hours -- hopefully something will break soon in the investigation and she is doing right by JD's loved ones by telling LE everything she knows about the week JD disappeared.