On your last note: Why over-complicate something simple? You're better than that. How about going into what types of keys + modes can mix together like C Ionian & F Lydian?
Of course (and not to be rude) I'm guessing you said the "overcomplicate" bit because you don't actually know what I'm talking about…?
The point was that using an enharmonic key does
simplify things. The most well-known example is Schubert's Moments musiceaux
number 6 (D. 780, Op. 94). It's in Ab major but at a certain point he wants to modulate to the relative major (Fb major) of the parallel minor (Ab minor). But Fb major isn't a key (you would have a Bbb in the key signature, which doesn't happen). So to get around that he wrote it in E major. So it looks funny to go from four flats (Ab major) to four sharps (E major), but it's completely logical if you understand the connections between the keys and how the enharmonic spellings are related.
As for mixing modes, mixing C Ionian and F Lydian is sort of meaningless. They both contain all of the same pitch classes. To "mix" them would be to somehow have both C and F sounding like tonal centers while still only using the pitch collection of C Ionian or C major.
True modal music will have one of those acting as a tonal center. The different intervals within the mode in relation to the tonal center or 1st scale degree of the mode are what gives it a "modal" characteristic in contrast to a pure major or minor tonal sound.
On the other hand, if you're talking about using modes in terms of chord/scale theory as is used in jazz a lot, then modes can be mixed in lots of different ways.
For instance a lot of players will use the Dorian mode over every minor seventh chord in a song, Lydian over every major seventh chord, and Lydian b7 (4th Mode Melodic Minor) over every dominant seventh chord.
This infuses some variety into the note choices as otherwise one might just be playing within a key, and thinking in modes isn't really all that helpful in that context. So if you had a ii-V-I in C major you could think of it as playing D Dorian, G Mixolydian, and C Ionian over the respective chords, but in the end you're still only playing notes from the key of C major (which goes back to your question about modes).