chronology and/vs fantasy
The problem of "the one interpretation" for this film depends whether you instinctively trust chronology (i.e event A happens B and therefore informs even B) or if you accept fantasy elements that negate causes-and-effects.
The fact that the last sequence repeats the first one but with a variation indicates that no simple exposition of situations to be resolved in chronological scenes can be trusted.
My thought at the time was to go with my "Pink Floyd The Wall"/"Donny Darko"/"J****'s L*****"") analysis.
Now get this (...this theory has hardly ever been proposed by "Donny Darko" viewers):
nothing actually happens between the start and the end of the movie
-which is very different from saying within the story-!
Everything we are shown is the product of a psychotic mind, the mind of someone about to die/in a coma who fantasises what could have been.
This elegant paradox allows us then to re-accept the last scene as being the meaningful closing of the story: the black magician in a coma (Trevor) IS catatonic and will NOT wake up from his internal hell (as he imagined he would in the first scene). He is, as they say, "locked in".
Now whether or not the manipulating -and highly unethical oh yes- doctor succeeded in getting through to his secret and discovering where he hid his magic book is open to discussion (the fact is we are shown a scene in which the doc declares victory).
...if you liked this movie, you may want to check out "Memento", "Reconstruction", Donald Trumbo's "Johnny goes to war" (is that the original title?), "Jacob's Ladder", "Twin Peaks Fire Walk With Me", that Italian animation movie ("Tornatore something"?), "Brasil", "le magnifique", "the butterfly effect", "leolo le heros" etc. etc.. + the "Lost Highway" / "Mulholland Drive" / "Inland Empire" trilogy.
+ of course (the master) Phil K Dick.
To be frank, I'm not completely convinced the film is a total resounding success but I certainly applaud its temerity; FFS let's have a bit of daring every now and then! Let's try to push the boundaries sometime!
And ten extra points for quite clearly shooting it on a ten pence budget. Respect to struggling and all the more passionate filmmakers!