After watching the drama for the second time, I see a clearer picture and agree with everything you stated above. After the grandma's poison, I too don't think ML has any trust issue with GTY. They talked about how he was upset that she didn't write to him while her sister in-law did. Her reason was that she didn't trust him to not get in trouble. Unlike her brother CB who had it easy since birth. CB's life was a smooth sailing while her husband had a difficult childhood and it's not an easy journey for him to reach his accomplishment hence why she didn't want to burden him with her problems. It's not that she didn't trust him but she is protecting him from any more grievances and burdens that he may encounters. It's her way of protecting him just like GTY chose not to involve ML in his grand scheme just in case if it didn't go well as planned then she would be part of their demise and punishment. In this case it's his way of protecting her and their family.