On the matter of Sergeant x Commissar attrition: Commissars are usually resented due to being foreign officers trained in the Schola Progenium and who are believed to have no sympathy for their troops (Knoll is from the same Forge World as the regiment, he was just trained in an offworld Schola) while Sergeants are usually liked because they are both soldiers like everyone else and the squad they are attached is already used to their Sergeant.
As for the Commissar's powers:
It is their duty to "encourage" the men of an Imperial Guard regiment to fight, fighting alongside the regiment's officers and taking control when morale is flagging or the men are not fighting with sufficient zeal. In such situations a Commissar can take over command from an officer and summarily execute any man who tries to flee.
Even a Colonel of a regiment must be sure to display courage and zeal at all times when a Commissar is watching them.
In general, they are political officers who can get away with a lot. Yes, Sergeants must salute them and yes, they can execute people and take command when such is deemed necessary. However, it doesn't mean they
have to do it. Preferentially, nothing goes wrong at all and the Commissar doesn't have to give actual combat orders. Most of the time he'll shout and roar to keep morale up to standards as well as, in some cases, try to inspire men through example rather than summary executions.