The Ethiopia argument is actually pretty weak.
First, contrary to common wisdom, there is limited evidence the "Queen of Sheba" actually was from Ethiopia. In fact, there is more evidence Sheba was on the Arabian Peninsula.
Second, there is no contemporary record of her son having visited Israel, if he existed at all.
Third, each of the Ethiopian Coptic churches has a replica of the Ark, and there is little reason to believe that the one claimed to be the true Ark is anything more than yet another replica.
I also think it is important to note that the Communist powers in Ethiopia are unlikely to have allowed a relic of this type remain undisturbed.
The most likely explanations for the fate of the Ark are that it was taken by one of the powers that invaded Israel, or it was hidden by the Temple priests.