Here's what I think about the possible involvement of others in addition to EA:
1 If there were any, EA certainly knows who they are.
2 If he wants to take the fall for all of them, and bear the brunt of justice alone, that's up to him.
3 If he gets worse criminal sentencing because he's seen as acting alone, when others were involved too, that's on him. He's bearing the consequences of his own choice.
4 If he doesn't talk, and there are others, it's probably going to be next-to-impossible to discover them.
5 If he doesn't talk, and he acted alone, looking for others is pointless.
6 My conclusion is that the surest way to get him to talk - and therefore to find out if others were involved too - is to simply prosecute him and let him face the consequences. Reality of the case against him may open his eyes and create incredible urgency as the clock ticks down on his window of opportunity for a plea that will close if she's found.