3) Is chaos corruption a slow and steady process or it can reach a critical point(like the black rage) from where you can't go back anymore (being conscious about reaching this limit)?
Much of chaos corruption is clinically the subversion and replacement of the subjects soul by a malevolent entity of the warp. Which can happen bit by bit, or catastrophically. Anyway, the subject has fallen to Chaos when the demon exerts control, effectively the subject is lost and all that is left is a flesh puppet hosting the demon. All sense of the previous personality is also lost, but the replacing demon may be a skilled pretender, especially if it happened gradually.
Now, before control is transferred, and some sense of the original self is still maintained, it's possible to renounce chaos. Even after the onset of initial mutation. This path is really difficult though, because once chaos has a hold on the soul it keeps sucking at it. There are some incredibly rare things that can purge the touch of chaos from a soul, like the gentle touch of the Emperor. Good luck getting your mutant an audience though.
Just as the warp reflects real space, real space reflects things in the warp. Mutations are an example of this, as the soul is subverted the demon warps the body to better suit its nature.
Civilian travel depends on the tech level of the planet in question. Might be by draft animal and wagon on regressed worlds.
Sororita are exclusively recruited from the Schola Progenum. Ergo they had to have been orphaned while the Imperium would still consider them 'minors' and joined the Adepta when they 'came of age', probably in some kind of ceremony or rite. So, most likely some where between 15 and 21 depending on the cultural traditions assumed of the greater Adeptus.