I had a very bad experience when I used to play the Qd6 Scandi with a6. It was when white opted for a kingside fianchetto. I found myself spending the entire game trying to avoid getting crushed in a highly complex middlegame completely favoring white the whole time. Not exactly what I call a user-friendly defense when white knows the g3 lines.
My two correspondence wins are in the 4.d4 Nf6 5.Nf3 c6 (I think 5...a6 is decidely inferior) 6.g3 Bg4 line. Both played in the trails of a Caruana- Tiviakov game.
Maybe Black has to play 6...g6 against that line, I'm not fond at all of Bg4/Bf5.
Qd6 4. Knb5 You might as well Qd8 No? Or never 2.d5 Isn't there so much better response to e4 then this? e5 or c5 has and always will be the answer to e4. It is known.