Well, first the 2.36" shaped charge was only 1.9 kg, not 3.3. So that reduces your calculation by a fair amount.
Not really. You're not considering how the weapon actually works. Its not a single bang, then a launch. It has a initial, short bang, then a short thrust. This is especially true in the case of the panzerschrek as the propellant charge used in it burned considerably slower than that of the M1 bazooka, hence the addition of the shield. Recall that at this point, you are still using a rocket, whereas the rpg-7, and AT-4 are not, they are classified as a grenade. The firing methods are similar but different. In fact, the RPG-7 and AT4 have a direct lineage to the panzerfaust, and NOT the schreck and bazooka.
Secondly, you can see the lack of a significant blast in any ww2 footage of the schreck or faust firing.
http://www.youtube.com/watch?v=iwInslkBnusWatch the whole video, but specifically note what happens starting at :30. The camera is actually directly behind someone firing an M1, less than about 10 feet away, and suffers no apparent harm. That's proof positive that the backblast is not lethal, and in fact not very evident at all. Later in that first film, you can see a soldier firing a m1 in a trench and no one in the trench or him suffers any apparent negative effects, if you tried the same thing with a RPG-7, or a AT4, or a SMAW, the shooter and everyone in the trench would have a very bad day indeed.