Irregulars like Hendo Lok Bloodmadder are certainly not human.Most of the irregulars we know are human while some like Phantaminum are not confirmed.
No.The children are regulars.Bam is an irregular but his case is complicated.
3.Irregulars bring chaos and change to the tower.Its not necessary that they are powerful.Its just that all the publically known irregulars are real powerhouses.So people are less likely to consider Rachel is also an irregular.Then again to change the Tower power is necessary.The tower is less likely to choose powerless beings.
5.Arlene is a pitiful character.She lost everything due to the tower.Jahad and the family leaders were nice guys when they were climbing the tower.Its the tower that corrupted them.We can see parallels with Bam (everyone around Bam says he'll change as he goes up the tower).The tower took everything away from her.Her husband,child, friends......everything.Its natural she wanted to escape.
6.They are the same.But only Headon travels extensively.