[Malachor Sith Temple (from star wars rebel) style to rescue Vette/Torian]

As far as I'm aware sith artifacts weren't used for that purpose. As far as I'm aware, the only sith in legends continuity that is known to have successfully been capable of manipulating time (which is what SWTOR is in) is Darth Ramage (a Baneite sith whose holocron plays an important role within Coruscant Nights and The Last Jedi 2013).

[6.2 another Disney land rehash boring storyline]

I personally liked how they ended Vitiate here more than KoTET (which I felt was rushed). They still shouldn't have brought him back, but at least what was there was decent. I think that characters such as Revan, Meetra, Kira, Scourge, etc should've had some level of help in regards to dealing with him in KoTET. Btw TRoS is a rehash of Dark Empire, which go the idea to bring Palpatine back from George Lucas (if you look at interviews from Tom Veitch, he discusses how the original plan was for someone pretending to be Darth Vader, which George vetoed and suggested bringing Palpatine back instead). https://www.starwarsunderworld.com/2016/08/interview-with-dark-empire-writer-tom.html