In general, you can’t get Rune magic by offering worship to a living demigod such as the Red Emperor, Jar-eel, Belintar, or Androgeus. You offer them magic points out of respect, self-interest, tradition, fear, loyalty, or whatever – not because you expect to wield some fraction of their magic. After all, they are currently using it!
If that demigod is no longer active in the mundane world (e.g., Red Goddess, the Only Old One, etc.) then it becomes possible to establish a path to their eternal self and wield some fraction of their magic through cult.
What about Belintar? The canonical take is that Belintar is no more – his “trick” for maintaining both a divine link and mortal free will is shattered and ended. There is no return of Belintar – and without him holding the links, the Holy Country loses its magical unity.
Is Ironhoof included in this list of demigods? Yes.
Is the Red Emperor a subcult of the Red Goddess? No.
Again the most basic explanation is – if you are operating with free will within Time, others can’t wield some part of you. You are still within Time, and not eternal. You can show others your path and they can try to follow it and perhaps gain some of the same power you did (or maybe not), but they can’t just gain a divine link to you and wield your power that way. That needs to wait until you are no longer subject to Time.
Note that demigods have plenty of rules they need to follow as well.
What about bringing the Only Old One back, is it being attempted? Who would do that? The fear is that he is returning anyways, as the Only Old One is viewed by most humans nowadays as a monster.