Well said as always, afitzy! I think / have thought all along that MT was in a dysfunctional relationship with a very controlling, powerful, charismatic, wealthy (in her eyes), manipulative man, and she just went along with anything he said or did. Whatever her reasons for being with him and staying with him through thick and thin (being kind, say she 'did it for her daughter' so she would have a 'good life', she likes rich and 'dashing bad boys', she truly loved him, was into him, wanted to blend their families together and become the matriarch), she's bound to be sorry now that her man has been caught red handed killing his wife, and she went along with whatever he told her to do.
He was such a scary and abusive husband to JD, I can't imagine he was immediately less scary and abusive as a 'paramour' to MT, although I'm sure there was a honeymoon period when all was rosy for them before JD's father died and she left him shortly thereafter under duress, and the shared bank account that JD's money went into was no longer available to fund FD and MT's lifestyle they had become accustomed to.
IMO, she's in deep, and hedging her bets on keeping her distance from FD for now, and seeing what they can get away with, or if it's looking bad for them, they can run from to a foreign territory that they already had planned out before JD's murder.
MOO