(Adds comment by Redstone's daughter, adds Viacom's closing share price, paragraphs 14-16)

By Dan Levine and Lisa Richwine

LOS ANGELES, Feb 29 (Reuters) - Media mogul Sumner Redstone's lawyers suffered a defeat in court on Monday as a California judge tentatively ruled that a lawsuit challenging his mental competency could proceed to trial.

The decision, by Los Angeles Superior Court Judge David Cowan, dashes an attempt by Redstone's lawyers to put a quick end to the case, which has raised questions about the 92-year-old's role in decision-making at Viacom Inc and CBS Corp.

Redstone controls about 80 percent of the voting stock, though he stepped down as executive chairman at both companies earlier this month.

Redstone's former girlfriend, Manuela Herzer, claims the multibillionaire was mentally incompetent when he removed her as his designated healthcare agent in October.

Redstone suffers from a speech impairment but was fully aware of his actions when he replaced Herzer with Viacom Chief Executive Philippe Dauman as the person designated to make decisions about his medical care if he is unable, his lawyers argue.

However, the judge questioned the arrangements under which Dauman directs Redstone's care, and his ruling clears the way for an extensive exchange of documents and interviews by both sides in the runup to trial, scheduled to begin May 6.

Herzer's attorneys are planning to interview Dauman under oath, as well as Shari Redstone, Sumner's daughter, who opposed Dauman replacing her father as Viacom executive chairman earlier this month.

While the judge said he could not make conclusions about Redstone's mental capacity short of trial, he did express concern at the state of Redstone's health, according to a tentative ruling distributed before a hearing on Monday in Los Angeles.

California judges often issue tentative rulings, which are then finalized after a hearing with few major changes.

"The court finds it perplexing that Redstone still puts Philippe Dauman and for that matter Thomas Dooley, the COO of Viacom, ahead of his own daughter as his agent in case of his incapacity," the judge wrote in the ruling.

"It has to be an unusual situation where a parent still at this late date puts his East Coast business colleagues ahead of an adult child, or for that matter adult grandchildren, in terms of his care."

The judge questioned how a CEO in New York, such as Dauman, had the "time or ability" to look after Redstone in California, even with the best intentions.

DOCTORS' VIEWS DIFFER

Redstone's lawyers argue that Herzer is merely seeking financial gain after Redstone's move in October to revoke a part of his estate plan that would have given Herzer a $70 million inheritance.