No. They could not be. What happened was best.

Had they been left alive the civil war could've drawn on longer with further intervention from the west.

perhaps, but it was an ongoing civil war where leaving one of them alive could have led to them being captured by the Whites which would have been a huge boon and rallying point for the disunited White movement. with Puyi it was different because the right KMT were republicans too and would have probably executed Puyi, especially since he had been a collaborator of the highest order with the Japanese. if he'd been captured by the enemy then it wouldn't have been potentially disastrous for the CPC in the way that capture of a Romanov by the Whites would have been for the Bolsheviks

Henry Puyi was one of the weakest leaders of the 20th century. He had no will of his own and went whichever way the wind blew. He was such a loser he couldn't even get a girlfriend. They took him to a girls' high school where he picked one of the girls to be empress. He ended up not only serving the Japanese, but ended up sweeping the streets (the lowest possible occupation) after China's revolution. He didn't even have the sense to escape to a comfortable retirement like any tyrant.