No, they were offered a pardon if they complied. They’d only be sent back if they fucked up.
No, the first offer merely offered them the chance to die in the line-of-duty. Lee Marvin had to fight for a highly conditional offer of pardon.
IIRC, the (at the time) extremely shocking and graphic bits were the non-censored nor discretion shot-ed burning people scenes. I haven’t rewatched it in a long time but IIRC a handful of Germans manage to break out of the bunker as Doomed From The Start Black Guy drops the grenades, which causes a few to become human torches.
Today that barely raises an eyebrow. Back then it was macabre, gore, sadistic and what have you. The times, they have indeed a-changed. I chuckle at the thought of that selfsame Ebert from that same era being shown the opener of Saving Private Ryan 
Right- at first, there was no promise of pardon or any other kind of reward. Lee Marvin convinced the general (Ernest Borgnine) that there had to be SOME hope of a commuted sentence or the men would have no reason to cooperate with him.
Reluctantly, the general allowed that IF any individual showed unusual valor or served with special distinction, he’d consider a commutation… but HE would get to decide who met those criteria.
Wladislaw (Charles Bronson), the only survivor, DID get a pardon.