Will the promised Kingdom still be called forever if the millennium ends after a thousand years?
Yes. The promised Kingdom remains forever even though the Millennium lasts a thousand years. Scripture does not contradict itself: the thousand-year reign of Christ is an introduction into the Eternal, and when eternity begins, it continues on the same basis, with Christ ruling and nations present.
Les explains that all through the Old Testament the Kingdom promised to Israel is spoken of as forever and ever, without end, while no time frame is attached until Revelation gives the thousand years, called the Millennium. He says the thousand-year reign of Christ is really just an introduction into the Eternal. Les is convinced that even though the millennial reign will end and there will be a complete change in the Creation and Universe, when we enter Eternity it will be on pretty much the same basis as the Kingdom: there will be nations again, and Christ will be ruling and reigning, he thinks from the New Jerusalem. So the thousand-year concept should not cause confusion about the Kingdom being forever; it will be forever. The thousand-year reign is another time of testing, because those entering the Kingdom are only believers—Gentiles who believed during the Tribulation under the 144,000's preaching, and the remnant of Israel will be saved.