We do not know how long they have been trying to avoid their transformations. It could have been only for a week so we have no idea how they were/could have dealt with the fact that Werebeast lose health when they do not kill or if they were successful in resisting the full moon. If that were the case it seems to me that that would have been an important point to put inside his journal. And Kodlak could be referring to the transformations in which they have a choice in, accepting the moonlit transformations and bloodlust as a necessity of their existence.
It would be nice if Kodlak dated his journal. But still, that he said "give up his transformations" doesn't imply to me "except those we're still forced into". If they were still forced to change, I'd have expected him to note it, and in particular comment on how it affects him given that he also says it "provided a clearer head" (in other words, wouldn't he say how forced transformations would affect his "clearer head"?).
I'm not saying you're wrong here, but I personally find it unlikely. His journal makes it sound like they willingly stopped changing altogether, not that they only stopped except when still forced to. There's also no mention of a werebeast being able to resist changing under the full moon without Hircine's Ring, that I know of.
Kodlak was not there when Terrfyg met the Glenmoril Witches. He has no idea how things went down and the notion he puts forward, that the Witches evoked evil magic, is classic Nord prejudice against Witches and magic in general which has been a well established theme in the series.
He knew enough to know Terrfyg had been tricked by the Glenmoril Witches, and that the power his group originally got was supposed to be temporary. Being that this all only started a couple hundred years ago, that would actually place it around the time of Arena/Daggerfall/Morrowind/Oblivion, so it's not unreasonable to think Terrfyg kept records of what happened.
Something I would like to know, though, is if Terrfyg had been tricked into permanently having lycanthropy, why would the Circle then make it a requirement? If they were tricked into keeping something they only wanted for a short time, why require it for new members of the Circle? The only answer I can think of is because they still had control over it. It was still a useful power and the drawbacks were minimal given what their honor called for. But honestly that's not a fully satisfactory answer.
We also have no idea what happened between the Wereboar and Hircine. It could have been similar to Sinding's story and situation. If the Wereboar was indeed not hunting then it would have experienced the same consequences as the PC while infected in Daggerfall. This was clearly not the case or whatever happened between Hircine and the Wereboar was enough so that Hircine didn't care to let nature take it's course.
In Daggerfall you wouldn't die from not hunting. You just got your max HP reduced until you killed an innocent person.
The Lycanthropic diseases have always been presented as just that, diseases.
Sanies Lupinus is more like a standard disease. It just makes you a bit weaker and can be cured using standard methods. But once it progresses to Lycanthropy, its nature becomes much more divine. Not only does it do something no other common disease can, it can't be cured by any common means and it changes your spirit to be more aligned with Hircine. Once you have Lycanthropy, it's not a normal disease anymore.
To me, the idea that there are also beast spirits in the package comes completly out of the blue because before Skyrim there was no such notion. And they are present only in one single quest and then a result of an artifact that no one has ever mentioned before. And it has been hinted at for a while that Werebeast could control themselves. If anything the Companions and Sinding confirm this making the idea that beast spirits take over redundant in my opinion.
I'm not so sure it's that a beast spirit "takes over", more than it affects your nature by asserting influence over it. Your original spirit defines who you are, then the beast spirit comes in and changes you. The beast spirit links itself with your original spirit, giving you the need and instinct to hunt, kill, etc. At times those needs flare up, and other times they're more subdued.