@Reverend I agree with pretty much everything you said, although we do have two Canonical instances of a Jedi using the Dark Side and remaining 'pure' (both coming from Return of the Jedi) that contradict your assertion that a Jedi can't tap into that power without forfeiting their status as such.
The first of these two instances comes at the start of RotJ when Luke force's his way into an audience with Jabba. The Gamoreans who are guarding the throne room try to impede him, and he Force-chokes them in response.
The second instance of course comes at the end of the film when Vader's words about Leia cause Luke to snap and he flies into a rage, only to mentally pull himself back from the edge and declare himself to be a Jedi.
The first of these two instances comes at the start of RotJ when Luke force's his way into an audience with Jabba. The Gamoreans who are guarding the throne room try to impede him, and he Force-chokes them in response.
The second instance of course comes at the end of the film when Vader's words about Leia cause Luke to snap and he flies into a rage, only to mentally pull himself back from the edge and declare himself to be a Jedi.