[Wmshyrga]

...can we truly know it is impossible? ...can we also see that this is an opinion?

In the context of the Dhamma we can indeed discover and know for ourselves that it is impossible. Without parsing this epistemologically (I tried that first but the length and detail felt out of place in this thread), we can simply examine the claim:

"Monks, it is impossible indeed, that one can pursue sense gratification without sensual desire, without perceptions of sensual desire, without thoughts of sensual desire."

1. The pursuit of any sense gratification requires the desire to gratify the sense(s).

2. The desire to gratify the sense(s) is the sort of desire that lies at the root of suffering.

3. The pursuit of any sense gratification is at the root of suffering.

4. The pursuit of an orgasm is the pursuit of a sense gratification.

5. The pursuit of an orgasm results in suffering.

(3) is a necessary conclusion from premises (1) and (2), (4) is a tautology, and (5) is an extrapolation based on (3) & (4). We would want to pursue the analysis along these lines. Taking your lead, we can ask: is it mere opinion?

Claiming that (1)-(5) is mere opinion is claiming that, at some point in the reasoning, there is no evidence, or the evidence does not prove what is claimed. You might contend, for example, that (2) assumes that desire lies at the root of suffering, and I would have to respond to that.

In short, the counter-claim that this is mere opinion needs to be backed up with its own facts, or the counter-claim falls to its own critique (as being mere opinion).

As an aside, please note that nowhere is reference made to the Suttas being perfect with no defects (whereas, for example, Theism usually rests on a circular chain of "the text is perfect and divine because it says it is".) It is simply not a claim we need to make when we are examining whether the Dhamma is consistent and functions as claimed.

Finally, this addresses your last point about people being on a path meaning they are more able to ascertain the truth-claims their path makes. This is actually a strong claim, and needs evidence to support it. Until there is evidence, it also falls to the critique of being mere opinion.

However, as you can see from the process above, anyone can potentially analyze the Dhamma with equal felicity, whether or not they are actually practicing the Noble Eightfold Path. "To be known by the wise for themselves," we might say.