Nearly all of these are simply mistaken statements. First, the Earth is NOT a sphere. This was hammered in to me by multiple physics professors during my grueling college experience (if you don't believe me, do a search on google for "earth is not a sphere" and you will quickly find what I'm talking about. Don't look at the Christian results - look at the secular results, usually notes from Physics/Astronomy courses). It is much closer to an ellipsoid, though that's not quite accurate either.
an IRREGULAR ellipsoid. with a rather small eccentricity. for all intents and purposes, the earth is approximately spherical.
Still think the ancient writers using "circle" couldn't mean "sphere?" Then you're just being intellectually dishonest.
no, i don't think it's out of the realm of possibility. we've all heard the two terms interchanged now and then.
HOWEVER it is my opinion that authors of at least the majority of the hebrew bible (and christian) were under the impression that the earth was flat. i could cite verse, but they could all be read other ways (as is the case with ANYTHING in the bible).