3a -edges. This is not such a great answer. One could have an infinite flat plane for example (thus being flat with no edges). Similarly, one could have a hemisphere (not flat but with edges).
I agree that "edges" is a weaker implication of flat-earth than the others - but I don't think it's unjustified. For one thing, the atmosphere is not purely transparent, so it should be impossible to see the sun through it if it were infinite in (horizontal) extent; for another, the sun has to come up somewhere.
These challenges could be evaded by videogame physics, for example, but such would be in conflict with human intuition.
The following is a series of exercises designed to test one's understanding of "Making Beliefs Pay Rent (in Anticipated Experiences)", a post in the Mysterious Answers to Mysterious Questions sequence by Eliezer Yudkowsky.
A. Examine the following list of statements. For each statement, ask: What observations would you expect to make if this statement were true that you would not expect to make if this statement were false, or vice-versa? If no such observations exist, indicate that this is so.
B. In the Dan le Sac vs. Scroobius Pip song "Thou Shalt Always Kill", one of the injunctions given to the listener is, "Thou shalt not put musicians and recording artists on ridiculous pedestals no matter how great they are or were." After this statement comes a list of such bands, beginning, "The Beatles: Were just a band. Led Zeppelin: Just a band. The Beach Boys: Just a band." Consider just this first statement, that the Beatles were just a band. What does it imply in terms of anticipated experiences?
The author's remarks on the solutions to these questions appear in this comment.