First of all, good analysis of the meaning of honesty! However, I see several problems with your application of this to the real world.
I think in most cases phrasing unintelligible for one group would not lead to them adopting any beliefs whatsoever. People also are not blank slates, just waiting for anyone to give their opinion on something, rather they have established worldviews and networks of beliefs, so your arguments might nudge somone already going in a particular direction, but will not make anyone believe something completly different. Also, I think you are overestimating the number of ways in which a statement can be interepted (I can not see how "The bill will increase the national debt by 12%." could be interpted in maéaningfully differnt ways. But feel free to give me counterexamples.)
You are correct that someone is unlikely form a new belief about how the world functions or something just because they hear someone say so. On the other hand, they're very likely to form new beliefs about you, and in politics, about your fellows and your constituents, on the basis of what they hear you say.
Some things can be very clear, sure. First, I'd invite you to consider what you would do, as they say, in the least convenient possibel world. If you did have to work around impossible communication barriers that forced you to either shut up or deceive somebody, what would you do?
As for the realistic nature of this supposed situation, it's very easy to show indeed. Just think how often politicians accuse each other of "cutting" funding to some vital thing because they've proposed not increasing spending as much as some previous plan.
That's a very easily understood ambiguity for a nerd. The next one is harder.
Suppose you say you support gun control. If you're not naive, you now know that people will make many assumptions about your other stances, on taxes, abortion, and so on. They'll use one answer to infer your ingroup, and use your ingroup to deduce your other answers. Your answer will, you know, make them believe false things about your other stances. Maybe the best thing to do is weigh how many true and false beliefs will be caused by any answer you give and try to rank them somehow...
The next is worse.
Realistically, no one person can be expected to be an expert on even half the issues that, say, the president of the USA will have strong influence upon. Honestly, candidates should probably say something like "I have no idea how to solve these problems. I have a few ideas about X and Y, but, honestly, superior experts can probably think of better ideas, and I should just listen to them. My only real qualifications are, my judgment in choosing which experts to guide me, the list of goals I will try to achieve with whatever power I achieve, and my ability to play the political game in order to make those goals happen."
Refreshingly honest, self-aware, insightful, and suicidal, wouldn't you say? Nerds, and rationalists, especially, would love to hear an answer like that. But, all the insight would be lost upon political-thinking normal people, and when that's been washed out, all that would be left over would be various admissions of ignorance, which would be interpreted as admissions of weakness and inability. There are a whole host of false beliefs that such an answer would cause, but the central one would be that this candidate would suck as president.
But the gulf between this truth and even a moderately acceptable answer is so huge that you pretty much have to lie if you want to win. You could always give up on winning, seeking honesty instead, but, as discussed in the previous paragraph, that would just lead to deceiving potentially fewer people.
"Well, you see, what I meant by that, see, is that I would do all in my power to ensure the bill would pass through Congress by being voted on, not that I would necessarily vote for it myself."
"What I meant by that..."
Is true honesty to speak the truth? Then were all ancient people despicable liars, as they were incapable of speaking the truth about the world, about which they were ignorant?
Well, then, is honesty to speak the truth as best you know it? So, the above politician, and those like them, are honest, because "what they meant" was some true statement?
Hmm...who's to say if you did "speak the truth as best you know it"? Is it based on how you privately intended the statement to be understood? Or is it based on how the majority interpret your statement? Or how the smartest interpret it? The people nearest to you? Or is it based on how you expected some particular group to interpret it?
This, I think is the real key: Honesty is to try and make your audience hear the truth.
Less important is whether or not you speak the truth. Who cares what you're saying; are you talking to yourself? The important thing is what they're hearing. So, shall we boil you up and throw you out the window if, despite your best efforts, others hear something you didn't mean for them to hear?
No, I think all we can ask is that you try your best to make them hear the truth, as best as you understand it. The honest person is this: the one who speaks words which they expect to make other people form true beliefs about the world.
You can fail to fulfill this principle in a number of ways:
I hope this idea of honesty makes sense. You cannot claim honesty on the grounds that what you said "was" true if you knew from the beginning that saying this "true" statement would make people form false beliefs about the world! If it were otherwise, you could purposefully make others believe falsehoods, and then call it "honest" because you had some other private, personal interpretation of your statement that you knew nobody would infer. If this is at all confusing still, then let's just taboo "honesty" for a second. Trying to make people believe falsehoods is bad. The fact that the wording you used to deceive people could also be interpreted in some other true way? Why on earth would that matter? You deceived people! On purpose! Call it honesty or not, but this is clearly a bad behavior.
You can only claim honesty if you try to make people believe the truth.
Ah, but now the great trick. Cast your mind out, imagine your message was going to be interpreted by millions of people, dozens of subcultures...in order to be honest, your message must be designed to make them believe true things.
Wait...make who believe true things? People are different, subcultures are different. A wording which will predictably make, for example, some nerds understand a true thing about the world may simultaneously be a wording which will predictably make some, for example, non-nerdy, blue-collar Latinos believe false things about the world...If you use that wording, and know that both subcultures will hear you...you know one will form true beliefs, the other false...were you honest or not?
Forget if you were "honest" or not; what are you supposed to do? It is literally impossible to say something to large groups of people without knowing that you're going to make some of them form false beliefs (then those people will call you a liar). There will be several interpretations of your statements, and some will lead people to believe falsehoods.
It's my impression that a lot of "lying" in politics comes down to:
Once you realize it is literally impossible to be honest in a certain very meaningful sense, that is, that you just can't say something that will mostly likely cause only true beliefs, you start to think about the best hand you can play with the poor cards you've been dealt. And once you come up with a few plausible closest-thing-to-solutions, you might suddenly recognize where you've seen those solutions implemented before.