The Symphony No. 1 example is a good parallel.
In performance (Nov 10 in Marienbad) I indeed did push the tempo for the "Meno" sections. The definitive clue was in the orchestral parts. The second occurence of Meno moderato is followed by Poco allargando and then Tempo I, which indeed suggest slowing down from Meno moderato to Tempo I, the slower tempo.
And lo and behold, in the orchestral parts it is written Poco allargando al.......... and then Tempo I.
The source of the confusion for me is that Sibelius' initial tempo is Andante molto sostenuto, but he then writes meno moderato, not meno andante. If he had indeed written Meno andante as in your Symphony no. 1 example, there would be no case for a dual interpretation.
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Symphony No. 5 has a similar place in the first movement, leading into the 3/4 Allegro moderato.
He writes poco a poco meno moderato al... and then Allegro moderato. As the original tempo is Molto moderato, it is quite clear that he means accelerando.
Again, the confusion in my mind was created by the fact that the basic tempo in the Swan is Andante, not moderato.