Yes, it can. A colleague of mine is driving his 5k Dell Display with the MB Pro.

Yes. I use the Dell display with a 2015 Macbook Pro, using dual Displayport cables. It works pretty well. I think you need the model with the AMD M370X GPU to drive an external 5k display.

I also use the Dell 5k monitor with a PC with a Geforce 980 GPU; it works surprisingly well in Windows 10. (You can even play some games at 5k ---- Civilization VI is really pretty at 5k!)

Linux support is woefully bad, however. I guess this will change as these HiDPI configurations become more common.