So is the brand what severs their connection to the Fade or is that just a means of marking them as having gone through the Rite? If the latter it would help Templars see who was "safe" with regard to possession, so they don't have to worry about them (or so they think); if the former then how does it sever the connection and why is their restoration permanent, since the brand is still physically there?
Much of the alleged "cure" is only theoretical. There are only two instances of it occurring in practice. With Karl it was only temporary and occurred when Justice emerged and came to the forefront of Anders' consciousness during our fight with the Templars. Once Justice had receded back into Anders' mind, the effect wore off. With Pharamond he was actually possessed by a demon. He claims this was not what restored him but came after he had already been restored, which suggests he was immediately placed at risk of possession by being restored. Then Rhys theorises that any spirit would do; however benign spirits are harder to attract and generally need a spirit medium like himself. I think it is the degree of uncertainty over this that makes Cassandra wary of just announcing "we have a cure for tranquillity" because the mechanism is not fully understood.
I feel this is one of those areas where the writers didn't fully think through the implications of this new plot twist because there do appear to be contradictions in the whole thing and the connection between how Seekers are made temporarily tranquil and how mages are made "permanently" tranquil is not clear. When Divine Justinia authorised the research on the effect on the mage's mind, she was trying to establish if it was possible to deny the mage the link to the Fade, removing their magical powers and attraction to demons, without also neutering their mind with regard to emotions. She also wanted to find out if it could be reversed. The answer to the second enquiry would appear to be that it can. The answer to the first enquiry would appear to be that the two go together; but I'd still like to know why? As I say above, dwarves are not connected to the Fade, cannot do magic, yet have emotions, exactly what the Divine was trying to reproduce with mages. Solas says that the elves post Veil seemed like tranquil to him but that suggests he does not appreciate how truly horrific tranquillity is because whilst the elves lost their magical ability, they certainly did not lose their emotions, judging by their reaction immediately following his action and the state of modern elves, some of whom can in fact still do magic.
So strengthening the barrier between the mage and the Fade will reduce their ability to do magic or render it null and void. However, why does it affect their emotions? That does in fact sound much more like the mage has been given a substance similar to qamek. Yet tranquil are not mindless drones. They have freewill, just not the emotional drive to use it and can still think very effectively in logical fashion, hence their single minded focus on the job at hand and their ability to produce enchantments, just like the dwarves, which both mages and qamek filled qunari mages cannot do. Also, from what we have been told to date, the effect of qamek is permanent, it destroys the mind and is not reversible. Mind you, they used to say tranquillity was permanent so who knows?
The situation regarding tranquillity could well be important to future plots. Dorian states that Tevinter uses the Rite to punish political dissidents and thus he was very interested in the possibility of a "cure" because he knew mages who had been subjected to it. So I hope they get the ideas behind the whole process sorted out because it does seem strange that it has been around for the best part of 800 years, possibly longer, is used in Tevinter to avenge oneself on political rivals and yet no one sympathetic to the victim had realised how simply it would be to restore them. You'd think with the free use of magic and spirits in Tevinter, with none of the restrictions down south, somewhere along the line a tranquil would have been accidentally restored as a result of a spirit "touching" their mind, even if their friends hadn't figured it out for themselves.