You know, I didn't write it but I also thought that someone used a massive magic event to fire their magic on top.
Your idea was Man-God started something and Orsted messed with it resulting in Nanahoshi's teleportation (Also implied by Rudeus)
I also thought that someone (Orsted) summoned/teleported Nanahoshi and Man-God used that massive energy to reincarnate Rudeus.

Why ?
Somehow Man-God can't act too much on the lower world. And somehow reincarnation probably needs a lot less magic, can be overlooked if something massive is happening at the same time and probably creates an hybrid from both worlds that is harder to erase (Nanahoshi's ideas)
Also, reincarnation probably is ok for a God, so less exposure. Even if in that case the reincarnation in another world is a transgression, even more so if the newborn keeps every memories from their past life.
And it was only possible because of the massive magic energy event.

Only my ideas though.