The card says: " Casts when Drawn
Take 3 damage. After you draw, shuffle two copies of this into your deck."
Casts when drawn causes another card to be drawn; this is still considered part of the draw sequence. So the card explicitly tells you that the reshuffled cards can not be drawn yet.
Not really sure what you say here. How did they have CB in their deck only and not in yours? Hakkar shuffles one in each deck?
It is possible to do an insta-kill after shuffling a single CB, but that requires several cards and good setup. The popular version is a Druid deck that wants to have Hakar, Togwaggle, and two times Naturalize in hand and their own deck empty. They play Hakar, next turn they get a single CB and 1 fatigue (3+1 damage) and then their deck is two CBs. At that point they play 8-mana Tagwaggle (swap decks) and two times Naturalize. Each Naturalize makes you draw two cards. If that doesn’t kill you, they pass the turn and you do another draw.
Here’s the math:
1st draw from 1st Naturalize: draw 1 CB, take 1 fatigue (6+1) - shuffle 4 CB.
2nd draw from 2nd Naturalize: draw 4 CB, take 2 fatigue (12+2) - shuffle 8 CB.
1st draw from 1st Naturalize: draw 8 CB, take 3 fatigue (24+3) - shuffle 16 CB.
2nd draw from 2nd Naturalize: draw 16 CB, take 4 fatigue (48+4) - shuffle 32 CB.
Pass turn, normal draw: draw 32 CB, take 5 fatigue (96+5) - shuffle 64 CB.
Total damage of the full combo is 201 damage, so you need to be at full health AND have more than 171 (!!) armor to survive.