Is financing through share tawarruq—in which a power of attorney is signed authorizing the bank to immediately buy and sell shares—considered permissible or forbidden tawarruq?

The correct method for "Tawarruq" (monetization) using shares is for the bank to purchase permissible shares and place them in its portfolio. Then, the bank sells them to you, and they are placed in a portfolio or account under your name. Subsequently, you sell them to a third party other than the bank. If the bank handles all of these operations, then this type of "Tawarruq" is not permissible.