First, a lot of people are indeed falling into solipsism. However, not everyone is, and not everything is "impossible". It is true that the barriers can sometimes be high, but they are never insurmountable - e.g., how hard would it be for someone to go get a medical degree? Okay, so that one is high, but there are other, much more low-hanging fruit! e.g. if a religious authority figure says that "nobody will die", and then a couple of months later, half the congregation dies, that does not need a decade's worth of study to figure out that the person "lied". Either knowingly or unknowingly.
Which brings me to point 2: if you can say the former phrase about lying unknowingly, then the definition of lying must be a bit broader than what you are using? People can be said to be "living a lie", as you said b/c they find out later - but perhaps even if they do not? Google's AI when I type in "lie" says:
used with reference to a situation involving deception or founded on a mistaken impression
So someone can be lying unknowingly if they pass on a statement that is itself a lie - and depending on the context, the punishment might not even be that much less severe, i.e. whenever the consequences are highly severe. But it varies with the level of "responsibility" aka the expectations set forth. Example: a nurse repeats word-for-word what they are told by a doctor to say - are they lying? Not really, especially if they are clear to attribute what is being done, in terms of merely "relaying" the message. The message itself may be a lie, but the person was clear, so is not a responsible agent for the deception, even if participating in it. But a doctor prescribing ivermectin on the other hand? They should have known better, and thereby for a person in such a position of responsibility to pass on improper information, may still constitute a "lie" in that case, even if an unknowing one - b/c they should have known. And if they did not know, then they should have found out. Others may need DECADES of study to catch up to them, but for a doctor who already knows the foundational framework, it is only a matter of a few hours to read some primary source material to catch up on exactly whether that drug is indicated in that scenario, and like what the side-effects are, etc.
In the above I had to make a major presumption here, in that someone did not pollute the various information streams that doctors have access to. Indeed if that were to happen, then it is possible for even doctors to, while passing on incorrect information, not be "lying" while doing so, in the same manner as a nurse. But I think at least that my former scenario is what happened during the pandemic? Someone started talking about using that horse drug, doing the work of a scientist except skipping the parts about actually doing proper testing, and so essentially doing unauthorized "human trial experiments" on actual, live human test subjects! :-( Perhaps they thought it was for the greater good even, like if people were going to die anyway then at least they could offer some protection? Except that's not even how that drug works under the most ideal conditions, thus doing so violates the most foundational and sacred oaths of the medical profession: to first do no harm. So then... it's a lie either way? Whether through nearly criminal ignorance or to fully criminal and unethical behavior. Tbf, not every "doctor" is a good one, BUT, in defense of my position, EVERY doctor (in the USA at least, and I thought in every part of the world?) MUST take the Hippocratic Oath. So it gets REALLY hard to defend such a person then, who either lied while taking it (in that they could not in fact manage to uphold those standards of integrity) or got lazy later on in terms of upholding it.
Which begs the next question: how can someone both "lie" and yet "not know that they are lying" at the same time? Admittedly this one is fairly complex in needing to dig deeper into human psychology. Or, I don't even think this is unique to humans, though it does seem far more developed in us than in animals. Let us switch scenarios b/c I think I have an easier one here. Let us say that a person has read the Christian Bible, and know for certain what the commandment by Jesus to "love one another" means - it means to be patient, and... you know what, let's just stop there. So when someone KNOWS that they have been COMMANDED to be patient, and yet they are NOT patient, but they still call themselves a "Christian" - that word means "follower of" btw - how then are they not "lying"? The answer, I believe, is that they are lying to themselves. Specifically, I am referring to cognitive dissonance: b/c our brains are complex enough that we utilize neural pathways that interconnect with one another without necessarily having to uphold one single, consistent Truth, it is fully possible for someone to both "know that they are lying", but also "not know that they are lying", at the same time. Such a person is usually LOUD in their condemnation of others who lie, and who e.g. are impatient, and yet they do not choose to see that they themselves are being thus. Hence the lie, b/c this is "knowing / willful misrepresentation of the Truth", the caveat being that here, only half of the cognitive processes are aware that it is a lie, while the other half act as if it is legit. These people will look you full in your face and claim that they are telling you the Truth. And that is the Truth. But it is also a lie.
2+2=4 | {2, 4} ∈ ℤ is a True statement? But if I say then that 2+2=2 | {2, 4} ∈ ℤ is also a True statement, is that a lie? What if I have no idea what those things ("numbers") mean? That gets back to that "accountability" issue from above - I really should know that, and all the more so if I am the one bringing them up? So acknowledging that and setting it aside, adding statements that are untrue converts a True statement into a False one. "There exists a True statement within this pair of statements" is True, but the overall pack of them is False. Hence, someone suffering from cognitive dissonance is guilty of telling a lie, to themselves. We all do it I am sure, it takes ENORMOUS efforts not to, especially when our culture is... well, as you mentioned, the way that it is. Though as we agreed: it is a descriptive statement to say that if and when that happens, those statements are still "lies", even if they are only partially known while partially unknown.
And all the more so when someone raises themselves up to become a (co-)leader of a nation - e.g. by voting. In that case, the statement that "they should have known" raises that specter, yet again, of responsibility: if they are going to chart the way forward for the entire nation - i.e. by depriving people of certain rights, like to medical care - then they should have thought deeper about the matter, and the excuse "but I did not know" does not work anymore. The reason it does not work anymore is b/c if you ever cross one of these people, they will cite this exact thing to you: YOU SHOULD HAVE KNOWN BETTER. It is the metric by which they judge - so it is not even me judging them, so much as acknowledging that this is the metric by which they judge themselves, and indeed by which we all judge our "leaders". At which point... they really should have known better, than to believe in a lie so hard that they actually vote on it, and all the more so when they do that in order to overturn the determinations of the people who actually DO know better - e.g. the doctors, who are aware that ivermectin is a horse drug, and if ever to be used in humans is only for extreme cases and for malaria, not covid and especially not as a preventative, and all the more so not as a substitute for a vaccine.
"Job titles are actually a fluid concept - why feel a strong need to label everything?" :-D