(written from a Production point of view)
National Amusements was founded in 1936 by Michael Redstone as the Northeast Theatre Corporation and has since gone on to become one of the world's leading media companies with more than 1,500 movie screens and "controlling interests" (an industry euphemism for de facto ownership) in CBS Corporation and Viacom, the holding company of Paramount Pictures.
As the conglomerate is fully owned by "media mogul" Sumner Redstone (son of founder Michael, and until 2016 also the co-Chairman of both Viacom and CBS) and his daughter Shari, this means that the Redstone family is presently the de facto legal majority owner of the entire, official Star Trek franchise ever since the franchise was acquired in 1994 by "old" Viacom, having itself already been acquired previously by Redstone in 1987. The position of identifiable individual legal majority ownership of Star Trek had previously been held by the Charles Bluhdorn family (1967-1994), and before that by Lucille Ball (1964-1967; Gene Roddenberry lost all rights and title to his creation the moment he sold his Star Trek is...-pitch to Ball's company, Desilu Studios, in April 1964 ). While the 2005 split resulted in the formal ownership transfer of the Star Trek franchise from Paramount to CBS, the ownership position of the Redstone family remained unchanged.
The 2005 split of ViacomEdit
It was Sumner Redstone, together with his trusted subordinate Les Moonves, who had engineered the Board-approved (on 14 June 2005) "old" Viacom split into CBS Corporation and "new" Viacom, which became effective on 17 January 2006, in essence undoing the merger between Viacom and CBS Studios that Redstone had acquired in 2000. His decision to do so – purportedly motivated by Redstone's desire to unlock the earning potential of the (at the time) faster growing soon-to-be "new" Viacom  – took the entire industry by surprise as it flew in the face of the general industry trend at the time of concentration through acquisitions and mergers, most conspicuously exemplified by the contemporary activities undertaken by The Walt Disney Company to achieve such.
Like Disney is currently doing (especially after the 2018 acquisition of 21st Century Fox), the National Amusements conglomerate is actually operating on the outermost edge of what is legally allowed. In 1948, the United States Supreme Court had its anti-trust ruling leveled against the motion picture industry – most ironically against Paramount Pictures in particular, being singled out by the Court as proxy for the entire industry – specifically aimed at the time to break the top-to-bottom vertical hold individual studios hitherto had on the entirety of the industry, otherwise known as the traditional "Hollywood Studio System" (see also Desilu Studios and Universal Studios in this regard). With full control over both production and dissemination of their movie and television productions, the conglomerate as presently organized – even after the 2005 split – is partially negating the intent lawmakers had in mind back in 1948.
Change of leadership and remerging effortsEdit
After her father was forced to step down in 2016, officially for age reasons, and she became his successor, Shari Redstone – said to consider the Star Trek franchise as a "family heirloom", and who had never been in favor of her father's 2005 split decision to begin with, but as then minority shareholder was impotent to prevent the move at the time – has been trying to remerge the two entities ever since. However, she immediately found herself embroiled in a vicious power struggle with both CBS CEO Les Moonves, co-architect of the split, and (new) Viacom CEO Philippe Dauman vehemently opposing the proposition, both of them tightly holding on to their personal fiefdoms.   
Having already managed to successfully oust Dauman as early as 2016 after a very unsavory power struggle complete with lawsuits and counter-lawsuits during which Dauman overplayed his hand,  Redstone found Moonves an even tougher nut to crack as he too took her to court in May 2018, seeking to diminish her influence over CBS.  However, less than four months later the struggle was unexpectedly and definitively decided in Redstone's favor when Moonves was forced to leave CBS on 9 September 2018 in the wake of the #Me Too movement controversies that he found himself in as of July that year when The New Yorker broke the news of his alleged sexual misconduct  – actually also unofficially part of the reasons why her father had to step down, as he too had not been entirely free of similar accusations. Part of the severance agreement however, was that Redstone had to cease her remerging efforts for at least two years. But while this at first glance might have looked like a concession on her part, it actually improved Redstone's chances of her mission to succeed, as it afforded her the time to purge the Board from Moonves loyalists, as is customary in such circumstances in not only Hollywood, but in other industries as well.   
Indeed, by early 2019, tentative steps were underway at CBS to effect Redstone's ultimate goal to restructure her company's holdings, which included the replacement of much of CBS' leadership.   If successful, this will for the Star Trek intellectual properties – split up between the two entities in the 2005 "divorce settlement" – mean a "reunification" of the television and film franchises. On 19 July 2019 Trekcore reported that Star Trek had even become part of the remerger motivation, citing a Hollywood Reporter article, which speculated the "reunification" to take place on 8 August 2019 at the quarterly corporate earnings reports, and that Shari Redstone's mission was close to succeeding.  It did not materialize on that date though, but a major stumbling block in regard to who should head the yet to remerged entity was removed, as the by Redstone favored Viacom CEO Robert Bakish was decided upon as such by both respective boards (even though the CBS board represented the much larger entity),  and beating out CBS head Joseph Ianniello, the interim replacement for Moonves, but whose position was still that of a temporary CEO only.  Not only that, but Ianniello is a known Moonves associate. 
Nonetheless, one week after the corporate earnings reports, on 13 August 2013, both companies made the official announcement that the remerger was in effect decided upon as well with Bakish formally announced as the overall CEO of the remerged and newly named ViacomCBS Inc. entity (essentially the resurrection of "old" Viacom), and with Ianniello's position officially elevated to that of permanent CEO of the CBS part.   Actually validating the July Trekcore report, both Star Trek and her franchise sibling Mission: Impossible were as the only ones in the official CBS press release specifically mentioned by name by the statement that the remerger "reunites fan-favorite franchises such as Star Trek and Mission: Impossible". 
Incidentally, and taking her cue from Disney, Shari Redstone's ambitions do not end when the remerger is effected, as she plans to embark on a Disney-like acquisition course herself afterwards, having her sights set on such companies as the film and television studio Sony Pictures Entertainment and the non-fiction TV company Discovery Communications (owner of the Discovery Channel – Disney owns the counterparts National Geographic Channel in full and The History Channel in part).