Well, technically I am closer to an IM than I am to an amateur, having two norms and all
FWIW, I can see the board absolutely clearly when playing a blindfold game. I can do it without great difficulty for 3 simultaneous blindfold games, but at later stages of the game I start to feel fatigue and need to focus really hard to continue. Of course, this is not 10 simultaneous opponents like Carlsen, or ~40ish like Gareev. But it is enough for me to tell you that this ability, as useful as it is for general speed and accuracy of calculation, does not mean that you can calculate 40 moves ahead in a middlegame position. I don't understand why you think that it does?
40 moves is probably about an average length of a chess game. The fact that you can ENVISION a full chess game, hell, 20 full games at a time, does not mean that you can CALCULATE it from beginning to the end.
You offer me to ask a bunch of 2400+ guys about this question - I retort with my own offer, for you to ask a bunch of 2700+ guys if they can calculate not 40, but 20 moves ahead in a middlegame position - you will be surprised. Hell, make it 10 and you will still be surprised by answers like "only in some positions".
In the field of calculation, what separates the pros from the joes is calculation speed and accuracy, NOT the depth. That's because there is a certain treshhold in depth through which the human brain just cannot pass and go deeper, we don't have enough mental capabilities. And that treshhold is certainly less than 40, by a mile. Sorry to dissapoint you, but Carlsen is not a super human.