Erm, if you agree to Torpor's deal, you can tell them you weren't strong enough to resist. Then the keeper defends for it being difficult to resist a demon's influence.
Is it possible it differs based on Hawke's personality? That is to say, diplomatic Hawke might say they weren't strong enough, while Snarky Hawke might say "Did I do that?".
Apart from there being no evidence to support this un-provable claim (why was Hawke not mind-controlled by Torpor? Idunna could do it), this would do nothing in favor of Solas's theory either -- if demons can mind-control you then neither they nor the Fade in general is safe no matter how you approach it.
Because in the lore, Sloth Demons can't use mind control. Only Desire Demons and Pride Demons, and both are loathe to use it if they can avoid it because they deem it crude. This is in a codex, in fact, and hasn't been contradicted to this point. Torpor is really only able to **** with peoples' minds once he takes control of Feynriel, whose power allows him such things.
Both Wryme and Allure make it clear that they're sufficiently pissed off at our meddling to not care at this point.
And Idunna is a person who said she learned her magic from equal parts Desire and something else.
Demons of desire are amongst the most powerful, and are the ones most likely to seek out the living and actively trick them into a deal. These demons will exploit anything that can be coveted—wealth, power, lust—and they will always end up getting far more than they give. A desire demon's province is that of illusions and mind control.
Strongest of all demons are those of pride. These are the most feared creatures to loose upon the world: Masters of magic and in possession of vast intellect, they are the true schemers. It is they who seek most strongly to possess mages, and will bring other demons across the Veil in numbers to achieve their own ends—although what that might be has never been discovered. A greater pride demon, brought across the veil, would threaten the entire world.
Codex Entry: Demonic Possession
The most powerful demons yet encountered are the pride demons, perhaps because they, among all their kind, most resemble men; as clever and manipulative as the desire demon, with a penchant for cruel irony that is almost human. While the demons of desire largely engage in the bribery of mortals, pride will use mortals' own best nature against them. Clever men outwit themselves. Strong men crush themselves. Humble men forget themselves. Jealous men fear themselves. They turn corruption and ruin into an art.
—From Beyond the Veil: Spirits and Demons by Enchanter Mirdromel
Note how it says a Pride Demon is just as clever and manipulative as a Desire Demon. A Pride Demon has the same level of mind control as a Desire Demon, confirmed separately here:
Desire Demon Codex
They are manipulated by illusions and deceit if not outright mind control, although these demons are reluctant to resort to such crude measures. Instead, they seem to take great pleasure in corruption. The greater the deceit, the greater their victory.
Only demons of pride prove more fearsome opponents when roused. Their abilities to affect the mind allow them to assume disguises and even alter the environment to their purposes, not to mention the great strength and speed they possess if they should have to resort to more physical means. Most often a desire demon will attempt to bargain its way to freedom if overpowered—many stories exist that depict mages defeating desire demons to the point where a wish can be wrested from them. It should be noted that in such stories the demon almost always gets the upper hand even when the mage thinks his wish has been granted.
The use of the words "more fearsome" is clear that they possess the same capabilities as a Desire Demon, and then some. So to say that the idea of the companions being mind-controlled has nothing going for it isn't quite accurate.
To sum up:
1) Three separate codex entries
2) the Companions saying they were mind-controlled.
As for Torpor, it's made clear that Sloth Demons don't necessarily mind control you so much as they sap your strength, your will, and leave you unable to care.
Which is why Merrill advises us to think active thoughts to combat it. So if Hawke fell victim to it, it's not because their mind was controlled... it's because their will was drained.
The truth is that demons of sloth are named so because this is the portion of the human psyche that they feed upon. Doubt. Apathy. Entropy. They seek to spread these things. The sloth demon hides in its forms, a master of shapes and disguises, always in the last place you look... and from its hiding place it spreads its influence. A community afflicted by a demon of sloth could soon become a dilapidated pit where injustices are allowed to pass without comment, and none of the residents could be aware that such a change has even taken place. The sloth demon weakens, tires, tears at the edges of consciousness and would much rather render its victim helpless than engage in a true conflict. Such creatures are best faced only with a great deal of will, and only with an eye to piercing their many disguises.
Sloth Demon codex