I've dark armpits, considering IPL for hair removal. Will it help lighten darkness? Should I seek alternative treatment? (Photo)

Yes.  IPL can not only reduce hair (although it is less efficient than other methods), but can also lighten skin.  We have treated clients with concerns such as yours numerous times.
I would suggest  IPL plus a lightening/bleaching cream used together for best results.  Make sure your practitioner is experienced with this approach however.

IPL/BBL can help with both of your concerns, but caution should be used. I would see a practitioner experienced in treating Asian skin to minimize the chance for any complications. We treat this issue in our office using very conservative settings with more sessions to prevent adding too much heat to the tissue at one time. We also use a multifaceted approach to improve the outcome.
I recommend starting on a lightening cream such as a hydroquinone or Brightenex from Dr. Zein Obagi. The Brightenex contains retinol which is recommended to help the lightening cream work faster by turning over the skin cells more quickly.
After the treatments, you may be instructed to continue a lightening cream occasionally and should wear sunscreen to prevent repigmentation.