Interesting ideas - the possibility that there were two Barad-durs, or a greater Barad-dur and Barad-dur proper, as it were, hadn't occurred to me. While I must say that I think it very unlikely that Tolkien had such an idea in mind when he wrote the Tale of Years, it is rather a nice harmonization of the sources.
Another possible explanation may be offered by a reference to Barad-dur found in 'Of Dwarves and Men' that I came across just now. Here Tolkien is discussing the Druedain and says parenthetically:
Quote:
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Originally Posted by Of Dwarves and Men
The tales, such as 'The Faithful Stone', that speak of their transferring part of their 'powers' to their artefacts, remind one in miniature of Sauron's transference of power to the foundation of the Barad-dur and to the Ruling Ring.
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Although a discussion of the relation between Barad-dur and the Ring is clearly not what Tolkien is focused on here, this statement does seem to present a different version of that relationship from what Elrond says. Namely, it suggests that Sauron transferred some of his own power into the foundations of Barad-dur and,
quite separately, also transferred some of his own power into the Ring. If this is the case, then maybe the link between Barad-dur and the Ring comes about not because the Ring was used in the making of Barad-dur, but because it was invested with the same power that was invested in Barad-dur. Instead of Sauron -> Ring -> Barad-dur we have Sauron -> Barad-dur and Sauron -> Ring. We might then take Elrond's statement either as an error on his part or as merely an imprecise way of describing the situation.
Of course, either of those explanations still leaves the 'Akallabeth' statement, which seems to place the fortification of Mordor between 1731 and 1869, in contradiction with the Tale of Years, which has it taking place from 1000 to 1600.