Maybe I'm missing something, but I thought the whole thing was that the wiretap itself was in fact legal? (Lester repeatedly talked about being on the up and up with it). It was only the EQUIPMENT that was missing, and that's where the manufactured serial killings came into play.
The Wire tap was granted for the cell phone of the alleged Homeless Serial Killer, not Marlo Stanfield.
I'm no lawyer, but I'll play one on the internets. I'm guessing Marlo's lawyer can get the wiretap evidence thrown out, since the wiretap was granted for the serial killer, not Marlo. Otherwise the police could get essentially get a wiretap on anyone they wanted by simply getting a wiretap warrant for one individual, and then using another individual's phone # on the wiretap.Also, remember at the end of the last episode how Daniels and his honey are in the evidence room and call the # to the cell phone which rings. I was sort of confused what exactly the purpose of this phone is btw -- was it used to route the "serial killer's" call through?, but once it started ringing Daniels and the lawyer both had that "our case is screwed expression" on their faces.
Wow. I'm not watching these careful enough. I guess I need to give it another go and/or stop watching drunk/late at night

I thought that was the phone they seized at the docks, and that it was "good" for the case that it rang.
After Greggs blows the whistle on McNulty's manufactured serial killer, Daniels brings the news to Pearlman. Piecing together the rogue detective's methods, they drive to evidence control to test the serial killer's tapped number against Marlo's seized cell phone. Pearlman dials the number off of a court document, and after a brief moment, the phone rings, confirming their suspicions.
And you're exactly right about not being able to just "swap numbers" on a wiretap, but I thought the Marlo one was authorized under existing warrants they had. I thought that's why they got 2 machines, hooked up on to the legit number on the phone used to call Scott and had that dude watching it with no activity, while Lester was monitoring Marlo's phone.And, maybe I should rewatch it, but I'm pretty sure the phone that rang in evidence control was a flip model, not the T-Mobile Dash that he was using for the pictures.I thought Lester was too smart for that... Now I'm disappointed.