From what I’ve read, if Halper is the man, he wasn’t actually embedded IN the campaign, but was trying to manipulate people - such as Papadoupolis - who were in the campaign, or on the fringes of the campaign. It sounds like he was trying to frame them or, at least, get them to perform actions or say things that would justify opening an investigation into the campaign.
A most excellent and very insightful comment!
If Halper is the guy, his role was to lure people from the Trump campaign and try to feed them false information. He didn’t have to be inside the campaign to do this.