Some apu's support crossfire with a dedicated gpu. It's like the poor mans crossfire. I wouldn't recommend it as its kind of useless, spending the extra money on a apu when you could just put the extra on your gpu.
cpu + gpu. The cpu in the apu is weaker and the integrated gpu is useless if you're going to use a discrete gpu anyway. On a side note I would recommend at least for a r7 260x for the gpu, and a r9 270x max to pair with that cpu.
The 860K is a newer design that the 760K and quite a bit more powerful . Use the 860K
What gpu you pair it with depends on your budget and your power supply
The hybrid crossfire set ups with a low end graphics card and an APU are marginal . Too flakey and inconsistent performance means its not worth the effort IMO