The trials that the Priestesses put Yoda through were done solely to make sure that he was worthy of learning how to preserve his essence after death, and effectively become immortal. Once Yoda successfully passed their trials, the Priestesses taught him the actual ability.
Yoda then taught Kenobi the ability, then it was passed to Luke, who passed it to Leia and Ben. Not sure who taught Anakin/Vader how to do it, but whatevs. /shrug
It's also worth mentioning that the Priestesses stated that Yoda: "Will teach one that will save the universe from a great imbalance. For this, the great gift will be his." That is the main reason they agreed to teach him in the first place.
Could be either of them, I guess. Technically, both were essential in bringing balance to the Force during RotJ(short-lived as that "balance" was.)
No clue when Vader would have learned the ability. Logically, it would've had to be sometime between him throwing Palpatine down the reactor shaft, and him 'dying' in Luke's arms... How in the f*ck he was able to master such an advanced Jedi ability in a matter of minutes is another story...
Lol, yeah. I mean, technically a spirit just giving some friendly advice to a Jedi would constitute "direct interference"... From a certain point of view.
Seems like a spirit's ability to directly interfere stops just short of them picking up a damn lightsaber and striking down the Jedi's opponent for them.
Maybe they chose not to interfere? It's possible they were more concerned about the futures of the force users than the political future of the galaxy.
I mean, if a spirit *could* directly interfere and actually help the Jedi fight their opponent or w/e, they certainly *would*(especially given what has been at stake in the past.)
Thanks to the ST, we now know that spirits can interact with the physical world on a level that none of us likely ever imagined(or wanted to see.) Logically speaking, it is entirely in the realm of possibility for them to: pick up a saber and have a full-scale duel with a Sith. OR TK throw ships at them. OR zap them with actual lightning.
So there must be some sort of cosmic 'rules' in place that they HAVE to abide by. One of which being that they cannot directly interfere to such an extent where they are actually fighting a Jedi's battles for them. Otherwise the immortal spirits of Yoda, Kenobi, Qui-Gon, Anakin, and Luke would have simply teamed up against Palpatine and destroyed him without Rey ever having to lift a finger. /shrug