By the time Glareanus "proposed" the Ionian (and other) modes, the modal system was already almost dissolved into tonal system.Gamma-UT wrote: Heinrich Glarean proposed the Ionian and Aeolian modes together with two other 'hypo' modes in his Dodecahordon expansion of the eight-mode system that started with Boethius. Glarean added them to address the fact that church music had already adopted their structures as they became more complex and moved further than Guido of Arezzo's hexachords.
The Ionian was, in effect, an altered version of the Myxolydian – substituting a B for the Bb that would have appeared in Guido's ''natural" (C-A) hexachord had it extended past A (La). In Guido's system there is no difference, because there is no seventh, and therefore no need for two almost identical modes. By Glarean and Zarlino's time, things had changed.
So, no, the idea that the Ionian mode never existed is false, as is the idea that Ionian intervals have nothing to do with the modern C Major scale.
Guido d'Arezzo method, based on the hexachord, was used as a mnemonic to help "modulate" from a mode to another (and there were three hexachords, not two - the natural, that goes from C to A - without B; the Durum that goes from G to E, with B quadrum; and ther Mole that goes from F to D, with the B mole). So, yess, Guido used the B also - but don't forget that in Guido method, the hexachords represented only intervals, not absolute pitches. When he signaled singers to change from the natural hexachord to the mole hexachord, he wasn't saying them to change pitches, but just interval intonations. BTW - Even now, in hispanic countries, the signal that lowers the notes a half-step is called "bemol", and the signal that raises a flat note half step is called "bequadro".
But it wasn't the only thing that helped the changing, and the transition from pure modality into tonality. As you pointed, the "musica ficta" practice already altered the modes that were being used (and were kept being used, in spite of Glareanus treaty) basically closing the gaps between modes, to the point that some of them become almost identical. Less than 100 years later, music was already tonal.
So, in fact, Glareanus "proposition" wasn't needed, because people used the modes in existance well aware of what they were and of what alterations were being introduced. But if you insist, when exactly would you say that this "Ionian" mode was used? Can you point any piece where it was used, and being firmly established?
Besides, a mode, whatever it is, has nothing to do with C Major (which is, itself, a mode, but also part of an entire new system of making music). So, again, the detail that thay share the same intervals is meaningless.