C#m7b5 is in fact the exact same chord as Em6, but with a different root. If we look at the notes both chords are built-up from, they are exactly the same (C#-E-G-B). Half-diminished and m6 chords are inversions of the same thing. You could call the same chord A7(9) as well, but it would need to have the A in the bass, because it's not featured in the chord itself. Similarly, if the bassist were playing D# (or Eb), it would be a Eb7(b9,b13). Half-diminished chords are very versatile.
In this progression there's no need to call it anything else than C#m7b5, because that's basically what it is. You could try substituting that chord with other stuff to find its function; in this progression to me it clearly sounds like a tonic minor (vi, from G) and you could play a regular Em chord there instead, making it the same as the very common I-V-vi-IV progression.
With regards to the 'Cadd9#4' chord; that name doesn't make any sense and I wouldn't call it that. To me it makes more sense to name it from G (Gmaj7sus4, with the C in the bass, in a Drop 2 voicing) or from D (D7(11), omitting the fifth, again in a drop 2 voicing). The latter makes the most sense, if you take the next chord into account, a D over C. You could view this as the bVII7 chord, going back to the tonic (although now in major) in the chorus.
I don't actually really think the band went full harmonic analysis mode when they wrote this though. They probably went with what sounded good (and it does), but you could explain it theoretically as well.