What happens to off chain transactions when the blockchain is reorganised?
You answered your own question.  Rephrasing as a statement:


  [Some companies] detect that Block 1 no longer exists and then reverse the transaction to A and B's wallet.  [Other companies] take the hit and lose the 10 BTC from their profits.


Which solution to implement is a choice that's up to the company---neither solution is inherently better or worse.  (Reversing the transaction is better for short-term profits, but taking the hit (if you can prevent most fraud) may make your customers happy and increase long-term profits.)

Bitcoin payments are push-based: Alice pays Bob.  An alternative solution to this problem is requiring depositors to provide a backup payment method that is pull-based. In the event of a double-spend, this will allow Bob to charge Alice's debit card.  Although expensive, time-consuming, and not guaranteed to work, the ultimate pull-based payment method is going to court and suing the double-spender.