I'm going to be honest, if my Quizzy hadn't had her hands tied by her Advisors all that time, things would have been run a lot differently.
If the Inquisition is disbanded and my band of rogue adventurers is just in my Inquisitor's hands? Solas isn't going to have a frickin CLUE what hit him.
Thats the thing. Solas knows how the Inquisitor as a figurehead and decision-maker, with thousands of lifes and the fate of Thedas on their shoulders, with hundreds of soldiers to command, thinks.
He does NOT know how a sole Inquisitor with new companions is going to act when free of the burden of command and on their own, say in a land that even a still-established Inquisition has no power in. P.s., corruption in Inquisition ranks will mean nothing if you're operating with a small team in a land without contact with your main group.
The only thing that Solas knows for sure, which he demonstrates in drawing the Inquisitor to him by way of the Qunari, is the fact that the Inquisitor generally stands for order and helping people. And that is honestly not much.
If anything, by going small, we'd be beating Solas at his own game: He's amassing hundreds, possibly thousands of followers. He himself says it: the fate of all organizations, to fall to corruption and betrayal. Solas is foreshadowing his own fate and the fate of the agents of Fen'Harel. Their size will be their downfall, while the Inquisitor can strike from the shadows in a small team.
EDIT:: Also, as for the mark. My standing theory is this:
Solas knew Corypheus had somehow survived the breach. I'm not sure how, but perhaps he has a connection to his orb and when he didn't find it in the wreckage he knew Corypheus must have it. Ergo, here he finds this survivor with the anchor; He knows Corypheus will come for it, that was his intention all along. However, without the orb, he isn't powerful enough yet to remove the mark and bestow it upon himself.
Rather than let the mark kill the survivor, Solas not only kept it from killing them, but also permanently bonded the anchor to the survivor, to keep Corypheus from taking it.
Unfortunately that binding was indeed permanent, and even after gaining more power, with his orb destroyed, Solas could not unbind and remove it. To save the Inquisitor's life he had no choice but to remove his/her arm. The way he did this, imo, is probably by turning the Inquisitor's arm to stone from the elbow down; His eyes flash everytime he uses that power, from Flemeth to the Qunari. Then the arm was either removed or dissolved by itself.