We have some more experienced people, and they will chime in eventually, but I figured I'd give it a try since people like to feel like they're not being ignored.

Caveat: I'm a beginner, enough so that I had to look up "acciaccatura". The definition I'm using is "a grace note performed as quickly as possible before an essential note of a melody, and falling before the beat."

My first thought is that anyone who asks me to play a grace note right before the main one that is the same note, is crazy. Given what a grace note is, and the capabilities of the human ear, I doubt it would be audible.

I'll assume you (or anyone else required to do this) fudges both the grace note and the gap between the two, and you're looking for the best way to make the fudging as minimal as possible.

My first thought, if being asked to do that would be a "stutter step", perhaps like one half cycle of a tremolo. This might be the same thing you were referring to as method 2, and it would definitely not be smooth. (But if you're playing the same note (on the same string), you'd have to lose some smoothness or the note isn't audible.)

To maximise smoothness and audibility both, I would say you'd have to use method 3. The difference in timbre would alleviate the need for a gap. The only big problem would be switching strings fast enough to qualify the first note as a grace note.

Method 1 should be considerably easier, and probably what people use when they can get away with it. In some pieces, the accent on the first part of the primary note might not be viable (when playing very soft or very loud, for example.) Then you'd have no choice but to use method 3 (or another method entirely).