Is it obligatory to pay the mall owner for the watch, knowing that it was won through a competition with "mentions" of some accounts, some of which are fake?

One must adhere to the conditions of the competition. If the prize giver stipulates that accounts must be real and active, then it is not permissible to use fake or abandoned accounts. Whoever does so does not deserve the prize, unless they inform the awarding body of the truth. Based on this, whoever used unreal accounts must return the prize or inform the awarding body of what they did.