I'm not sure why he didn't try out the Eluvian and Well in the Dark Future. Or maybe he did, but it didn't work. After all, in the Dark Future, he's likely won the Rebellion and the Templars, and the Inquisition is too weak to stand against him. The Sentinels struggle against his forces when the Venatori and Red Templars have been cut down to a fraction of their former strength. Or, I guess Abelas could have destroyed the Well and the Eluvian before the Temple fell. *shrug*
The Anchor allows the wielder (and others nearby, apparently) enter the Fade physically. The Breach was probably meant to sow chaos and serve as a gateway. I'm curious if the explosion was intended or if it was the result of the Inquisitor's interference. If the Anchor was burned into Corypheus instead, would he retain it through his body-swap, or is it specifically imprinted onto his body? Solas's comments almost make me think it's more of a 'soul' thing, a connection made in the Fade. That's all ambiguous, technical stuff though.
The Breach does wonders to cause chaos in the South, which makes his conquering of it much easier. So he nips into the Black City, gorges himself on Blight power again, and then uses the Anchor to seal up the Breach once he's created the Retro Tevinter Imperium in the South? Or keep it steady so that he can easily maintain armies of demons as enforcers?
All of this just reminds me that Corypheus was a High Priest of Dumat rather than a master strategist with a clear agenda. 