I personally think it’s possible and even likely that MT did not actually know about the plan and that FD may have intentionally misled her into thinking that he was at home, etc.
Based on close experience with someone like FD and seeing how FD treated others, JD of course but also former clients and workers, which is very familiar, FDs relationship with MT was likely “put up and shut up” and “don’t ask don’t tell” on a wide range of matters. She was a guest in his house after all which gives him enormous power, and to keep that lifestyle there’s a certain price for her to pay, with a vindictive and volatile guy like FD.
FD easily could have told her the bags were construction waste that his worker brought back in the Raptor from NC and he wanted to clean up before the weekend, etc- his other dumpsters were full, etc. At the time she might not have thought anything of it because JD had not even been declared missing yet, MT wasn’t thinking anything about JD when they were on the trash run. Although MT learned about JD missing the next morning when FD excitedly told the GAL, the news about the bloody bags only came out with their arrest warrant which was a whole week later.
I think it’s possible that MT may have told LE about the truck exchange with the employee. MSM worded it that FD “was driving his employees truck the day JD went missing”. That info came out the week after MT met with LE. Things might have started falling in place for MT after finding out about the bloody items in the trash bags, and of course her arrest.