From the point of view of a rager:
1) Is magic good or bad when used by Immortals?
Magic is so powerful it can ONLY be used by Immortals. It's quite fine then.
2) If it's alright, then why are so many ragers sore about Twist the god of magic?
Personal preference. He oversees a sphere of Magic, which no mortal should embrace. He should instead reserve that sphere only for Imm use. He doesn't, thus meh.
3) If it's not, then why are rager imms using it?
It's fine for them to use Magic.
4) How is a mummified mage head providing them with powers?
Gods of Battle have imbued the head with their power, a sliver of which a villager can call upon. It provides a channeling mechanism if you will for a mortal to use Imm power specifically approved and sanctioned for villagers.
5) Why are communes not allowed (other than game balance)? They come from the gods?
Communes are allowed in the village (village healer) and in the shrine of your deity. A proper villager is full of pride and doesn't need the stinky powers from Imms or who knows where and from whom.
6) Well magic can come from the gods, too.
Gods use magic, but it specifically comes from the plane of Magic beyond the Veil.
7) Besides, gods created Thera, so one way or another, everything comes from the gods.
Sure, but gods can do whatever, however mortals should be limited and not play gods themselves (i.e. use magic)