So, my first question would be, what are you trying to accomplish?
I ask this because you want to disable the "one finger chord", but then you apparently do not want to play 3 or 4 note chords, so I'm just trying to get some clarification as to what you are wanting to do with your accompaniment.
With that said, here is some information that may help. Please keep in mind that I have an older PSR-E433, but I believe the auto accompaniment on the EW410 works the same way...
With the auto accompaniment switched on, then the preprogrammed musical background, or auto accompaniment, is automatically determined by the notes you play on the left side of the keyboard. By that, I mean that you do not have to select whether you want an automatic "fingered mode" or a "one-finger mode" -- it's determined automatically by the keyboard by the notes that you play.
So, as an example, with auto accompaniment switched on, if you just play a single C on the left part of the keyboard, then the keyboard will assume "one-finger mode", and will play a full orchestral background based on a full C-major chord. However, if you play C, E, and G together (the notes of a C major chord) on the left part of the keyboard, then the keyboard will see that you are, indeed, playing a full C major chord, and it will still play that same orchestral background based on a full C-major chord.
Going beyond that, the "one-finger mode" is designed to play a minor chord if you play the note of the chord and the nearest black key below that note (so in this case, it is not really "one-finger" but instead, two fingers). So, if you play a C and the closest Bb below that C together (with auto accompaniment switched on), you will get a background based on a full C minor chord. And, you will also get that same background if you play the actual notes of a C minor chord on the left part of the keyboard, which would be C, Eb, and G (and in any order/inversion, as long as all the notes are played on the left auto accompaniment part of the keyboard). In other words, the auto accompaniment of this keyboard is smart enough to know if you are just playing a single note or cluster of notes that activates the "single-finger mode" or easy-play mode chords, or if you are playing the actual notes of the chord. When you play the actual notes of the chord, the auto accompaniment of this keyboard will recognize major, minor, 7th, and minor-7th chords and play full, orchestral backgrounds based on those chords.
There is also a sort of "hybrid mode". With this, you turn on the split voice -- this allows you to play a different sound on the left side of the keyboard than the sound you play on the right side. For example, you can have the main voice be piano, and the split voice be a guitar, so that the notes you play on your right hand will be a piano, and the notes you play on the left hand will be a guitar. Now, with split mode on, you can also turn on auto accompaniment, and now when you play the notes on your left hand to get the full orchestral background in the way that I described above, you will also get the sound of the split voice (in this example, the guitar) based on the notes you are playing. And what is really cool about this is that, with the auto accompaniment on and split voice engaged, when you do the "single finger mode" chords, the keyboard will automatically not only play its preprogrammed background based on the chord, but will also play all the notes of the chord with your split voice (the guitar, in this case), as well. So that, if you play a C note, not only will you get the preprogrammed background based on the full C major chord, but the keyboard will also play a full C major chord with that guitar sound, as well.
The benefit of this is that you can add rhythmic chords with the split-voice sound of your choice to go along with the preprogrammed orchestral background that the auto accompaniment plays with the style. So, with the example I just said, where you play a single C note and get the automatic preprogrammed accompaniment with that C chord, if you rhythmically or repeatedly hit that C note, you will also get your guitar sound repeating every time you hit that C note, while the automatic accompaniment continues to play in the background. And, of course, this also works if you are actually playing the chord: In this example, if you play the actual notes of a C chord rhythmically and repeatedly, you'll hear the guitar sound each time you hit the chord while the preprogrammed orchestral accompaniment continues to play in the background. This gives you some control over the background you are playing, even when you are using the auto accompaniment.
If you do this, you'll likely want to raise the octave of the split voice by 1 or 2 octaves so that the split-voice chords generated by the auto accompaniment are not low and "muddy" sounding.
Now, if instead, you do not want the auto accompaniment to play any of these preprogrammed orchestral backgrounds, you can switch auto accompaniment off. But in this case, it will still play the drum rhythms when you select and turn on a style. And in this case, the keyboard will just simply play whatever notes you play on it, like a piano, regardless of where on the keyboard or which part of the keyboard you play those notes. So, with the auto accompaniment switched off, the style will play the drums, but it will not fill in automatic chords, backgrounds, and bass lines like it does when the auto accompaniment is switched on.