[Ardamir the Blessed]

I finally found something in the Akallabêth that could be a sign of it being written in a Round Earth context:

...at times, when all the air was clear and the sun was in the east, they would look out and descry far off in the west a city white-shining on a distant shore, and a great harbour and a tower. For in those days the Númenóreans were far-sighted; yet even so it was only the keenest eyes among them that could see this vision, from the Meneltarma, maybe, or from some tall ship that lay off their western coast as far as it was lawful for them to go.

Why would one see more clearly from a place high up if the world was flat? And also:

And men saw his [Ar-Pharazôn's] sails coming up out of the sunset, dyed as with scarlet and gleaming with red gold, and fear fell upon the dwellers by the coasts, and they fled far away.

This reference is similar to those I have found in 'Aldarion and Erendis':

Thus it came to pass that on a morning of fair sun and white wind, in the bright spring of the seven hundred and twenty-fifth year of the Second Age, the son of the King's Heir of Númenor sailed from the land; and ere day was over he [Aldarion] saw it sink shimmering into the sea, and last of all the peak of the Meneltarma as a dark finger against the sunset.

At last the sea and wind relented, but even as Aldarion looked out in longing from the prow of the Palarran and saw far off the Meneltarma, his glance fell upon the green bough, and he saw that it was withered. Then Aldarion was dismayed, for such a thing had never befallen the bough of oiolairë, so long as it was washed with the spray. "It is frosted, Captain," said a mariner who stood beside him. "It has been too cold. Glad am I to see the Pillar."

Also from 'Aldarion and Erendis':

he [Meneldur] was enamoured of the stars and the heavens. All that he could gather of the lore of the Eldar and Edain concerning Eä and the deeps that lay about the Kingdom of Arda he studied, and his chief delight was in the watching of the stars. He built a tower in the Forostar (the northernmost region of the island) where the airs were clearest, from which by night he would survey the heavens and observe all the movements of the light of the firmament.

Would he not discover that Earth is round then?