I'm not an expert, but I can offer some thoughts on the matter.

What Solas said in Haven must be true. When most people dream their initial surroundings reflect the place where they fell asleep. When Felassan dreamed in TME everything was the same besides the haze of the Fade. When you first reach Skyhold and approach Solas you're dreaming. (Which could mean was either spying on you or just fell asleep right as you approached...) Anyway, the game cuts to Haven, but I think it's likely that Solas led the Inquisitor on a simplified path he created to his dream mock up of Haven. Solas is a dreamer who all seem to have exceptional abilities in the Fade. In DA2 Feynriel was in Tevinter, but was able to use his skills to communicate with and defend a woman in Kirkwall. It's likely Solas really only needs to physically be in the ancient ruins to interact with the spirits/memories lingering there. He can recreate the ruins, but not the memories. Without the memories he can't see what transpired in the past. So yes, Solas can shape, control, jump into another dreamer's dream. It's also likely he can travel the Fade to reach another dreamer in a matter of seconds even if they are physically hundreds of miles apart. Who knows where he physically was when he confronted Felassan.

Oh, wow, I never considered the Fade. I'm probably wrong, but from what I've interpreted about the ancient elves, they lived during a time when the Fade was intertwined with the real world. If that's true then the Fade was never it's own separate location. Maybe Solas was amazed at how it had adapted on its own? Maybe he was the same way when he first woke up the physical world. I love how he starts geeking out. At one point he even sounded like a tour guide like "[Okay kids! Buckle your seat belts!] And prepare for what is certain to be a fascinating experience." Shouldn't he have been a bit more sad at the state of the Fade? It's a dark dreary place filled with corrupted spirits. Who knows, maybe his ancient elven eyes see differently than normal people. He and Sera see the sky differently... Or perhaps were they looking past the Breach and into the Fade? Hmmm...