From on your key statement "It appears (...) my supervisor has this grand idea already fleshed out (...)[and is] unwilling to hear that some statements might have to be toned down a bit" my understanding is that you are perceiving potential flaws in your supervisor's thinking.
Firstly, my congratulations, as you seem to be maturing as an independent thinker, ie. a scientist. You are probably what I would call a 'true' PhD student, which (I am afraid) might be not so common nowadays.
I believe being skeptical towards colleagues' ideas is fundamental to healthy scientific logic. This is discussed in the invaluable books from Bachelard, Popper, among many others. The problem is, this is not in line with the practice and expectations of modern academia, where a sense of "collegiality" and smooth political connections are valued over traditional logical and scientific rigor. A main message here is: a maturing PhD student ought to understand the dividing line between scientific practice from academic practice. Contrary to what ivory tower inhabitants preach, science is not contained within its walls. I discuss the two sides of this coin below.
Side 1: Questioning your colleagues and being skeptical until convinced otherwise is healthy for your scientific formation. Constantly debating with your peers should be an essential part of your formation as a PhD student, towards becoming an expert in your field. In this line, I encourage you to politely push your advisor into discussing your project with you in open terms. Bring in papers, seek the opinion of others in the field. Come up with your own hypotheses and encourage others to question you.
Side 2: It is common practice in today's academia that institutions compete for available resources within a logic akin to the corporate environment of companies in the real-world market. Departments try to impress an internal sense of political unity focused at short-term materialistic goals (e.g. authorships, grant proposals, equipment, etc) for institutional survival. This favours a mentality where criticism and judicious thinking are counter-productive. Disagreement becomes an impediment to fast publication instead of the origin of new ideas. Advisors are seen as office heads and staff managers, and students/technicians/postdocs jointly regarded as employees to generate quickly the assets. Questioning an advisor equals insubordination which in corporate logic leads to micromanaging and firing (i.e. alienating the student and failing his degree).
My personal experience furthermore suggests your advisor (others here might say "your boss") has quite personal ambitions regarding your line of research that he would like to see published on paper. Under his name. However, unable to achieve this personally from his bureaucratic seat, the supervisor expect "his staff" to produce in his behalf. That would explain why you're not allowed to question his will, against any logical thinking. Because this person wishes his ideas published as such.
Finally, my advice is that you weigh inside yourself whether you value science over academia or vice-versa, and what is the current tradition in your institution and among workmates. Prioritise your objectives as strategically as possible to minimise conflicts of interests. Stay aware that successfully acquiring PhD training does not equate to successfully earning a title. Consider postponing the open reasoning and publication of your ideas if you wish to work as a true scientist inside the academia, while already being conscious of the necessity of finding the best like-minded colleagues and collaborators.
Sorry for an intricate answer, and good luck!