How many flat-earthers are just pretending about their beliefs in order to screw with the rational and/or show that they’re not part of the common herd?
Kyrie Irving is an example of such a person.
Some of them don’t know how long a mile is. Misapprehending time delays because of the speed of light would be a doddle for them.
Not really because firstly that doesn’t prove man went to the moon - could have been left there by an unmanned probe. And secondly you need a gazillion watt laser and a highly sensitive receiver. Watch the video of the Mythbusters being shown this experiment. It’s way beyond what even a serious amateur astronomer can do.
Flearthers don’t have those things themselves and even if they attended a demo they’d just say “how do we know the positive result isn’t fake” since it involves trusting hardware and software they don’t control.
Stand between a set of train tracks & look down; they are wider than you are; now look out in the distance; they appear to come together; except if you walk out that distance they are the same width as where you started. Distance does funny things to your perception.
Pizza is (relatively) flat & round; that view of the earth is just like looking down on a pizza.
Now, if we combine my two ‘proofs’ we can get all of the flat earthers having a pizza party on the train tracks…problem solved. 
Constant, you say? Mwahahahahaha!!! Where is your god now?!
I’m gonna quit with the Cellular Cosmogony thing now. I am, however, glad I did the shtick for two reasons
Numero Uno- It was fun.
Numero Two-o - The search for Cyrus Teed’s book led me to the Internet Sacred Text Archive. ISTA - Internet Sacred Text Archive Home The site is a true treasure and beyond fascinating. In a few seconds of searching the Archive, I found both Eddas, The Niebelung saga, the Kalevala (spelling?) and English translations of the Talmud! And it’s all free to download! Highest possible recommendation.
To some extent nothing will convince them, since if you take them up on a rocket and show them that the earth is curved, they’ll say it is distortion from the windows. Or, no doubt CGI projected on the windows.
Besides the flight mentioned in the OP, they could charter a ship and circumnavigate Antarctica. Forget about them not seeing the ice walls - I just saw a video where they explained the cruises that take people there from South America and New Zealand as going to the only two places without ice walls, conveniently located by chance near embarkation points. But if anything works the laser should. Not for the plain deniers, but for the Ferfers.
The whole flat earth thing fascinated me for a while. I cannot understand why a functioning adult can believe such stupidity. I’ve come to the conclusion that they do not. They revel in the attention it gets them. It is essentially performance art.
For the earth to be flat there has to be a conspiracy involving every ships captain, every pilot, every surveyor, everyone in IT that works with GPS, everyone teaching science, doing science.
The Cold War has to be a giant hoax,
The list is endless.
So there has to be this ginormous conspiracy that has their members in perfect lockstep to achieve… what? What is the goal of this fantastic enterprise?
And then this perfectly functional conspiracy is not capable of shutting up a couple of YouTube channels? How does that work? Do they have some superpower?
They have to know it it bullshit.
I have occasionally “admitted” to conspiratorialists that yes, there is a conspiracy, and that everyone is in on it, except for them.
You’re lucky to have never known anyone like this. I think one of the appeals is supposed secret knowledge. These people, all basically losers, like to think they know things we sheeple don’t. Makes them feel superior.
Also, I suspect that logical thinking skills are distributed across the population like anything else, and these clowns are in the bottom 10%. My friend can’t draw logical conclusions even in things having nothing to do with politics, religion or geography. It’s just a talent she lacks. I can tell this from her absurd book plotting.
There are ostensible Flat-Earthers who don’t believe it. They get their jollies by making up elaborate “explanations” of why the proofs of the spherical world are wrong and similar things. I’m not sure I’d call it performance art, though. The rest of the Flat-Earthers are dumb enough to believe the crap the first group makes up.
That’s essentially what most CT comes down to for me: the desire for the person espousing it to feel like they have some “secret” knowledge or able to see through the bullshit that us “non-thinking” peons can. It’s an easy way to feel superior, especially if one has a lackluster life. The instant anyone uses the word “sheeple” unironically (not in your context), they lose any credibility with me. When it comes to CTs, I just nod my head, say “that’s interesting” and try to change the subject. They cannot be convinced. Even if the one in a thousand can, for my own peace of mind, it’s the easiest way to handle them.