Hi, Paolo:
As Herbert said, midi editing is time consuming. On the other hand, far less so than learning and recording a new instrument, so there is a trade-off.
To edit attack and release of notes, go to Piano Roll, type "ep" to enter "Edit As Performed" and select the notes you want to edit by click-dragging.
When notes are selected, the sub-pallete of 9 icons will become active, letting you shift the "attack" (i.e. when the note begins to sound), the "release" (i.e. when the note stops sounding), and the overall duration of the note. You can also snap selected notes to the nominal default attack/release/duration.
By judicious editing, you can shift notes slightly before and after the beat.
If the built-in arpeggio "strum" is too much, you can also edit the attack and release of the arpeggiated notes with the method described above.
One important thing for realism is to figure out the actual rhythm pattern of your "strum" and alternate up and down arpeggios (arpeggii?). You can then copy and paste those into the appropriate bars to minimize editing.
Here's another piece where I used the techniques above. "I Remember Friday Nights (In '71)" is from my musical The Day We Flipped Mike's Chev. An old timey rock-y roll-y thing-y.
David