People, it's really very simple.
In chronological order:
ACOS is one song with multiple movements. Most of the time it has been played as one song, but on occasion, individual movements (or parts of movements) have been played on their own. But it's one song.
WFS/LTL are 2 songs. They are not now nor have they ever been one song.
AMBI is three songs, not one. It has never been presented as anything other than a 3-song suite. They have been performed all together, separately, and with only 2 of the 3 together. But they are three songs, not one.
The Mirror and Lie are two songs. They have been performed together and separately. But they are two songs, not one.
SFAM is one album of 12 songs. They are organized into 2 Acts, and 9 Scenes, but they are 12 songs, not one.
SDOIT is one song of 8 movements. Most of the time it has been performed, it was the entire song, but some of the movements have also been performed individually, much like ACOS. The only difference between SDOIT and ACOS is that the movements on the SDOIT CD were individually tracked for convenience. But it is one song, not eight.
ITPOE is one song, not two. It was written and recorded as one song. The vast majority of the times it has been performed live, it was performed as one song, although the sections were played separately a time or two. The only reason it was ever divided was that the band felt that it was the song that would serve as the best album opener as well as the best album closer; therefore it was split for presentation on the CD. But it is one song, not two.
The 12 Steps Suite is a SUITE (as it has always been referred) of 5 separate songs written over the course of 5 separate albums. The intention was that they would eventually be played together live as a suite, although I would imagine that this particular ship has sailed. All of the first four songs have been played individually (and yes, TGP and TDS have been played as a pair). But they are five songs, not one.
The label "epic" is a separate discussion, but I personally don't think that a song can be called an epic just because it is a certain length, or just because it is divided into separate movements. It is basically a song-by-song thing.