Other contacts in the prison? Stole one off a guard?
Ask the criminals in Real Life who have smuggled worse into prisons.
This. There have been inmates who have smuggled handguns into prison and kept them hidden from guards.
Some of the "high-value target" retrieval bonus objectives in Blacklist don't make any sense. Specifically, the HVTs in the Qods Force HQ in Tehran and the one in the Airstrip in Mexico. While all the other HVTs can plausibly be expected to be recovered because they are either found in America or in places where friendly authorities are operating, in both of these locations, there are no assets nearby to retrieve the HVT. Sam and Briggs are the only Americans operating in Tehran during that mission and they certainly didn't bring the HVT with them in their van, and the attack on the airstrip was something that caught the team completely by surprise, and they were in much more of a hurry to escape since they were on an unauthorized mission. Yet in both instances, after you bag & tag the target, you still get the "retrieval is on its way" message for them.
The CIA may have some undercover agents in Tehran that could do the retrieval. How they are able to sneak into the HQ of the Quds Force while they are on high alert and drag out an unconscious guard before his friend find him is another question.
Although this got averted by the time of Conviction and Blacklist, how exactly does Sam keep getting lucky enough to find enemies to interrogate that speak perfect English in the early games? Somehow I doubt a low-class, random Indonesian militant or a North Korean soldier stationed in some backwater base is going to know enough English to respond to Sam's inquiries.
There are only three Indonesians Sam interrogates: One of Sadonoís officers at the embassy, a technician in the submarine base and Sadono himself. Itís not a big stretch to find three English speaking people in a large guerrilla army with several hundred members.
This. These are the only interrogations you need to do - everything else is essentially non-canon.
So the Blacklist attacks: American Consumption = contaminating water. Okay. American Fuel = an attack on a tanker full of fuel. Fine. American Blood = an attack on the American top brass hiding in a secret bunker. These all make sense. But what does putting nerve gas on a bunch of trains have to do with American Freedom?