The reason for a slash in a note is not that it is a grace note per se, but that it is to be played outside the context of the meter. In other words, it's a note that you "throw in" in whatever way sounds good to you. So, it's that a grace note is played outside of the meter, so it has a slash in it, not that because it has a slash in it, it's a grace note.
So, the common interpretation is just as Jomiddnz has it, and as C.G. mentions, that is how it is played in pretty much every major recording of the piece.
Another thing not to make too much of is the fact that the D natural and E flat are written as 16th notes, which would take up the entire value of the 8th note trill. All it's saying is to put a turn at the end of the trill. (Don't expect those "little notes" to precisely follow a meter; they usually don't.) So, the figure is generally played like this:
In the usual tempo range, most people just put a simple mordent for the trill as I have it here. I've looked at several recordings, and nearly all of them definitely use the notes I have here (as do I). I'd say that you're entirely safe doing the same.
Also note that most performances borrow the grace note's time from the previous sub-beat, meaning that the first Eb of the quintuplet falls directly on the second beat, and the grace note a little before. This appears to contradict your wikipedia link for the acciaccatura, which says this:
In the 19th century, the acciaccatura (sometimes called short appoggiatura) came to be a shorter variant of the long appoggiatura, where the delay of the principal note is quick.
In the performances I've listened to, there is no delay of the principal note at all, and the grace note borrows from the previous beat. I believe that this is the correct interpretation of the notation; it's certainly the one that pretty much everyone uses.
