MISSOULA — A federal judge in Missoula planned a hearing Monday on a motion to end the W.R. Grace trial, in which the company and five former executives are accused of knowingly allowing human exposure to asbestos. Defense attorneys last week filed a motion calling the trial “meritless” and urging that it end. The motion seeks dismissal “with prejudice,” meaning the same charges could not be brought again. The defense alleges misconduct by the prosecution. At issue is the revelation that federal prosecutors withheld evidence that could have been used to discredit a key witness. Grace and the former executives are accused of allowing asbestos exposure related to the company’s operation of a vermiculite mine near Libby. There are residents who blame asbestos for widespread respiratory disease and dozens of deaths in the northwestern Montana community. The mine closed in 1990.