A dispute over the leadership of Island Landmarks, a Vashon nonprofit that owns the historic Mukai Farmhouse, is slated to go to court next month, where both sides hope a King County Superior Court judge will make a swift decision in their favor.

The newly constituted board, chaired by Islander Glenda Pearson, filed a motion for summary judgment last week, arguing that the new board should be considered the rightful leaders of the nonprofit because it carefully followed Island Landmarks’ own bylaws when it installed a new slate of officers.

But the court could also opt to remove Mary Matthews, who founded Island Landmarks more than a decade ago and contends she is still the legal head of the organization, because of her behavior over the past several years, according to a 23-page brief filed by Vashon lawyer Rex Stratton.

Matthews and her husband J. Nelson Happy have “privatized” the nonprofit, co-mingled the nonprofit’s funds with their own money and “engaged in conduct that was dishonest and damaging to Island Landmarks,” Stratton contends in his brief. “As long as Mary Matthews and her hand-picked board are involved with Island Landmarks, the Mukai property will never be the community asset that was originally envisioned.”

But Matthews and Happy, who now live in Texas, also had their lawyer, Islander Robert Krinsky, file a motion for summary judgment, in their case arguing that the Islanders who have put themselves forward as a new board did so by way of a “secret scheme” and in violation of the organization’s bylaws.

Only the organization’s treasurer, for instance, can deposit checks into the Island Landmarks’ bank account, Krinsky says in his brief. Thus, when Islander Ellen Kritzman, a former member of the Island Landmarks’ board, deposited several checks from residents into the Island Landmarks’ bank account, those people did not become members and could not hold a special meeting and vote Matthews and Happy off the board, Krinsky said.

“Those putative ‘members’ lacked the authority to hold a special meeting,” he wrote in his 10-page motion, filed with the court on Aug. 31.

The dueling briefs are the latest in a tug-of-war over the property that will likely be decided in King County Superior Court. The motions are slated to go before Judge Monica Benton on Friday, Oct. 12. The board headed by Pearson issued a one-page update about the legal case, urging interested Islanders to attend the court hearing.

The briefs — and hundreds of pages of supporting documents — add details about the dispute over the property, a long-simmering issue on Vashon. Matthews, for instance, said in a declaration that she and Happy “have personally advanced more than $300,000 to pay the operating expenses of Island Landmarks.”

But Stratton, in his brief, outlined signs of what he called a “dysfunctional” organization. Matthews’ board hasn’t held a regular board or annual membership meeting in the past 10 years, has ignored requests from Islanders to become members and has let both its federal and state nonprofit status lapse, he wrote.

He also said Matthews and Happy have used the Mukai property for their own personal use, staying there when they visit Vashon, and cannot account for $150,000 in public grant money awarded to Island Landmarks to purchase and restore the historic site.