Silent pic does take a bit to process. So yes, very small intervals will not keep up.
Forget about Canon powersave stuff. ML powersave will be the best you can get. The screen needs to be activated for the time silent pics does it's process. So you can shut the LCD off, but it needs to activate again for silent pics.
You can't put the camera into full powersave (sleep) mode. It obviously needs to be awake to do stuff!
AutoETTR needs the LV active to work. So even if AutoETTR gets enabled with raw video, the LCD must remain active.
The only current advantages I see for using raw video for an intervalometer is that you can have very short intervals, and the LCD can remain fully deactivated. The cons are increased processing time and currently a lack of deflicker options.
IMO, raw video and intervalometer are 2 different tasks. Raw video shouldn't be hacked to provide the features of an intervalometer. If the intervalomter is lacking, it should be improved.
Intervalometer is not lacking, silent pics is. With the current implementation of silent pics, it doesn't offer any advantage over RAW video. It is, in fact, an inferior RAW video.
As for LCD screen on for Auto ETTR to work, if it has to be done, then I'll do it. Because that is still not a disadvantage over silent pics, since silent pics require LCD to be on anyway (unless your intervals are long enough).
Either way, would memory hack not be enough for Auto ETTR to work? I don't quite understand how it all works, but I always assumed that RAW video is something like an extension of silent pics, where the camera captures the footage from the live view buffer. If memory hack can enable the LCD to be off (I'm assuming here that live view is technically still active, just that it is not displayed to the LCD) while recording, then it should also be able to make Auto ETTR work with the screen off.