My original point was not to break down the construction of the bombs as such, but to ask the question....how could the scenario suggested by some conspiracy theorists work? That is, where do you find "actors" (some of them amputees!) who are prepared to stand next to any explosive device as it detonates? If there is indeed, no such thing as a harmless bomb, it represents a fundamental hole in the theory. Another possible explanation would be that the area was cleared of the general public, the devices were detonated and the performers were brought in and the stage set (complete with blood and debris etc,) in the space of a few seconds.
I guess that in theory, you might be able to find such people, if you use a container that pulverizes so no dangerous fragments are released, cleverly designed hidden body armor, some sort of internal ear protection like wax maybe. Secret services all over the world have killed people, interrogated, with the help of medical personnel etc... very few of those have come forward to tell their story.
But apart from the "actors", you would have to have the cooperation of the first aid teams, the police, the hospitals, all the services that were present at previous editions. So either you have to recruit all of them, and it's highly unlikely that none of those will talk, or you have to replace them, something that wouldn't go unnoticed.
The surviving victims who have been identified in the press would have to change doctors, or their doctors have to be involved as well: if you show up in the media with a broken leg, your doctor will expect scars at your next visit. Same goes for all the friends, neighbors, family members, people from the swimming club, ...
You'll have to have lists of the hundreds that got hurt, a list that will exist for years and years, and at any time a reporter may decide to do an in-depth story about one of them. If they get into an accident, medical files have to be consistent. It's just a logistical nightmare of enormous proportions. Should anything go wrong, maybe they could use the FBI witness protection program as excuse to explain away discrepancies or hush it all up, but still...
And that doesn't include the risk of anything being exposed accidentally with all the cameras and mobile phones around. Or members of the public trying to help the wounded and noticing the kevlar.
Let's face it, such a scenario is fiction, if you want a false flag operation, you use a few people with real bombs. The idea of an all powerful government that lies and deceives, controls its citizens, kills people with immunity but spends enormous resources because it doesn't want to risk the lives of a few spectators at a marathon, that just doesn't make any sense.