I'm going to write this as if you have already watched the final season...so if you don't want
spoilers, pretend not to see this post.
[polite spoiler space]
So anyway....the reason Greg seems to disappear from the plotline in season 14 has to do with Mary's (actually Loretta's) true motives for what she was up to. She didn't care about Meg in the slightest. "Wanting to visit/know her granddaughter" was the pretext for worming her way into the MacKenzies' lives (especially since Meg was not really her granddaughter, but her grand-niece). Mack developed another of his hero complexes, trying to solve "The Mary Problem" all by himself (not even letting Karen help, for the most part) with one ill-conceived scheme after another. Even if he had been in the market for assistance, Greg was one of the last people he would have asked since they were so estranged at that point. I'm sure all the people arrayed against the MacKenzies in this plotline (not only Loretta and Joe, but Treadwell and the big boss Abby) were counting on Mack NOT getting Greg involved, since like you said, he could just "make her disappear" like he did before. I'm sure Loretta did not need to be told twice to steer clear of Greg. I assume the main reason they had that scene was just to make Greg look like a prime suspect when she turned up dead.
Loretta and Joe had been hired by Treadwell/Abby to distract the MacKenzies and keep them from being any sort of help to Paige with Sumner Group issues. Abby's plans to take over TSG hinged on Greg continuing his self-imposed exile from the company, since having Mack/Karen, Paige, and Claudia co-running things made the company ripe for a takeover. Greg had been trying very hard to distance himself from Meg and her parents, so the four of them were not interacting mostly by Greg's choice. Greg probably did not know what Loretta and Joe were up to earlier in the season since he'd relocated to Montana, and Mack's secretive scheming against Loretta kept most everyone (not just Greg) in the dark.
Then there is the complication with the brother, who I can't remember for the life of me how he was involved with Val's kidnapping and Treadwell....
Val went to Florida to follow up on some information about the real Mary Robeson (though she did not know at the time that Loretta was not the real Mary). Val discovered the real Mary had died in childbirth (we assume giving birth to Laura), and that her sister Loretta was impersonating her. Someone in Florida alerted Joe that Val was snooping around their family history, so it was decided to kidnap Val (Abby would never have sanctioned Val's murder) in order to keep her from exposing Loretta and Joe's scheme, since it was integral to Treadwell's over-arching plot to take over the Sumner Group. At first it appeared the man and woman holding Val hostage--friends of Joe's who were being paid off by Treadwell--were only going to hold her as long as it took for the takeover to occur, but then Gary "found" them and caused the car accident that made it appear Val had died, when in fact the two who died were the kidnappers. I assume this is when Val escaped, slowly making her way back to KL in disguise since she thought Treadwell would send more goons to recapture her. In the meantime, Joe killed Loretta, then one of Treadwell's goons killed Joe (so much for honor among thieves) and other seemingly unrelated characters (Nick, Vanessa) were revealed to be participants in this umbrella plot.