Joel Kline, the former Bethesda "boy wonder" of the white-collar crime who has repeatedly avoided long jail terms by cooperating with the government, has turned up as a key government witness along with self-styled "Happy Hooker" Xaveria Hollander in the prosecution of prominent New York architect on tax evasion charges.

Kline, a convicted stock manipulator whose tangled financial empire is in bankruptcy, was used as a phony consultant by New York architect Leo Kornblath and then returned the payments he received - minus a fee - in cash, according to the U.S. attorney's office in Brooklyn.

Kornblath of Roslyn Heights, N.Y., is president of a New York architectural firm that has designed interiors of major commercial buildings in New York, Washington, Puerto Rico and Europe - including part of the twin skycraper World Trade Center in lower manhattan.

According to David G. Trager, U.S. attorney for the Eastern District of New York. Kornblath not only made "fictitious" monthly payments to Kline in Washington, but also to "consultant" Xaveria DeVaries, also known as Xaveria Hollander, in New York.

According to Trager, Hollander also turned the money back to Kornblath in cash. She did it. TRager said in a press release, because she needed to be able to show apparently legitimate income to cover up the illgitimate income to she received from operating a brothel. Hollander also is expected to testify in the forthcoming trial.

Charges against Kornblath - three counts of filing false corporate Tax returns and five of aiding and abetting three others in filing false tax returns - all revolve around acts allegedly committed in 1970, 1971 and 1972.

Only Kornblath was indicted, following four-month grand jury investigation.

Becuase they are cooperating, Kline, Hollander and three New York architectural and construction figures were not indicted according to the federal prosecutor's office in Brooklyn.

THe three are Irving Gershon, a former partner in an architectural firm that was one of the World Trade Center's principal designers; Howard Lazar, executive vice president of a large New York construction form, and SDy J. Miller, president of a New York architectural firm.

Kornblath allegedly arranged for Miller and Lazar to pay Kline for fictitious services never rendered according to prosecutor Trager, after which Kline would send the money back to them incash, minus a "laundering" fee.

In Gershon's case, trager said, Kornblath allegedly arranged for Gershon to make "false charitable contributions" to the Eileen and Joel Kline Foundation, a family foundation that enjoyed tax-exempt status.

Kline served 4 1/2 months in prison after pleading guilty in 1974 to federal obstruction of justice charges stemming from a Securities and Excahnge Commission probe of stock manipulation. He files for bankruptcy that fall and hadbeen cooperating with federal and local prosecutors in several investigations of political corruption and wgite-collar crime.

Kline's testimony is credited with helping to send former Anne Arundel County Executive N. Dale Anderson to jail. He also aided the investigation that led to the resignation of former Vice President Spiro T. Agnew.

Last February, Kline ran a small advertisement in the Washington Post promoting himself as a "consultant" with no other explanation. Today, he can be reached only through a Bethesda post office box and a Sarasota, Fla., answering service. He did not return a reporter's phone call yesterday.