Through character creation and discussion with my players, I've come across some problems with Insanity, Corruption, Mental Traumas and Malignancies.

First, reducing Insanity and Corruption. Basically, to remove one point of Insanity costs 100 x ISb, and you can't remove a point if it means your ISb would reduce (ie. 40 Insanity to 39 Insanity). To remove Corruption, you need to get it prayed away by a priest or something similar; however, there is no restriction on reducing your Cb. This means that (RAW) Johnny the rogue psyker can have his daemon wings miraculously healed, while Billy the war veteran is stuck with bad dreams for the rest of his life.

Secondly, some Mental Traumas' and Malignancies' effects rely on ISb and Cb. For example, Psychosomatic Curse gives a character the Weakened (ISb) condition for 1 round if they fail a test by 7 DoF or greater. Now, it makes complete sense that the Weakened condition severity would increase as ISb goes up. On the other hand, Unnatural Limbs grants the character the Extra Arms (Cb) trait. It doesn't make sense that the character would gain even more limbs as they gain corruption, especially as they'll likely be gaining other malignancies as well - possibly even the same malignancy (which adds 1 arm through stacking). Many of the malignancies that are tied to Cb are in similar situations. In the example of Unnatural Limbs, I'd suggest changing the effect to "This character gains the Extra Arms (X) trait, where X is the character's Cb at the time of gaining this malignancy."

Finally, both the Mental Traumas and Malignancies tables have entries for 30. To get a result of 30 you need to roll two 10s on your 2d10 and have a ISb/Cb of 10. The problem is, to have a bonus of 10, your Insanity/Corruption must be 100+, in which case your character needs to be retired, as per p251.

Regarding Extra Arms specifically, I read it as a one-time acquisition that gives you one arm per your Cb at the time that you first get the malignancy. If your Cb goes up, you don't gain more arms. You only get them if you reroll the malignancy. I'd assume then that you would not lose extra arms by reducing your Cb, since once you have them you are locked in.

For that matter, I'd apply that universally to alll malignancies. Even if your Cb drops (or you clear your corruption entirely), it doesn't get rid of your malignancies. Once you have them you keep them.

As for the 30 rolls, I assume that they put those sections in to allow for certain special effects based on environment or other modifiers. For example, you might be in a heavily warp tainted area that imposes a +5 to your malignancy roll or something.

Regarding Extra Arms specifically, I read it as a one-time acquisition that gives you one arm per your Cb at the time that you first get the malignancy. If your Cb goes up, you don't gain more arms. You only get them if you reroll the malignancy. I'd assume then that you would not lose extra arms by reducing your Cb, since once you have them you are locked in.

This is how I interpreted it, too; however, the book isn't exactly clear - especially since it mixes one-off bonus checks with recurrent checks, without any distinction.