So a few observations...
I will say you were smart in not letting that dealership work on the car. If the turbo is bad, then why change out oil lines and water lines? If oil got in the water (or water in the oil) then there are bigger issues. If there is no water in the oil (or visa versa) then there is no need to change out those lines UNLESS one is damaged. Even then, you only need to change that ONE line. The chances of all four going bad all at once would be so astronomically improbable you have a better shot of winning the lottery...twice.
Exhaust manifolds can crack and they can cause issues with the turbo. I would have had them show you where the crack is before they change anything. If the crack is in the divider inside the manifold where the turbo bolts on to the manifold, this is totally normal, affects nothing, and I have yet to see an LNF exhaust manifold that is used without the crack.
Obviously a bad turbo can cause low boost especialy the levels you're talking about, but when a turbo goes "bad", it's done. Bolting it back up isn't going to make it work again. The bearings inside get sloppy, air leaks past them, and you have almost no boost, usually oil somewhere it shouldn't be, and usually smoke in the exhaust.
Now that I've covered the major issues they said were wrong but probably weren't, let me hypothisize what really happened.
That error code and your symptoms give a strong indication that you had a boost leak somewhere. The Engine Control Module (ECM) has two sensors that can detect boost. These are Manifold Absolute Pressure (MAP) sensors. One is located on the pipe that runs from the intercooler to the throttle body on the driver's side of the engine bay. The other is located on top of the intake manifold. Only these two sensors would be able to tell the ECM that the pressure level they are reading is lower than what the ECM knows it should be. If the ECM is getting a pressure reading less from these sensors (and really it's the one in the intake manifold the ECM is listening too...the other one is also a temp sensor and the ECM only reads the temp off that one) than what it expects, then it sets that code you saw.
Now, the only place in your system where you will see boost starts at the turbo (naturally), goes through the hot side charge pipe (silver pipe on the passenger's side of the engine bay), to the intercooler, then through the hot side charge pipe (where your first MAP sensor is), to the throttle body, into the intake manifold, and finally the cylinders themselves. If you have a leak ANYWHERE in those areas, you will loose boost.
We already covered how a turbo can leak and not make boost so I'll skip that. Also, a leak in the throttle body or intake manifold would be extremely unlikely. I haven't heard of one developing in those locations yet.
The pipes themselves would have to be physically punctured to leak and I've NEVER seen that happen outside the car being involved in a major accident. However, the silicone couplers that attach the pipes to the turbo, intercooler and throttle body...THOSE are usually your number one location for a leak. The clamps that hold them on (especially the OEM ones) if they are not tightened enough can come off or allow air to slip by causing a leak. If you heard a POP of some sort when you lost boost, chances are it was one of these couplers. Each coupler has two clamps (obviously) and there are four couplers so there are 8 places you could have an issue.
The OEM intercoolers CAN develope leaks but this isn't super common (but common enough) and happens almost exclusively to cars who have turned up the boost to make more power (even with the tune from GM Performance Parts or GMPP). Again, once the intercooler starts to leak, it leaks. At 14 psi, you are WAY under those boost levels known to cause the intercooler to fail. Factory stock boost levels usually only get to the 18 psi range. Tuned cars will see 20-25 psi and it is at these levels where we have seen some OEM intercoolers fail.
There is, however, one other component that can cause this error. This is your waste gate actuator solenoid. This solenoid is bolted to the front of the turbo and has three vacuum lines attached to it. The ECM controls this solenoid to regulate your boost levels. IF this solenoid fails, your boost levels will drop. HOWEVER, they will NOT go to 1-2 psi. They will drop to 4-6 psi. The spring in the actuator this solenoid controls will mechanically control boost pressure up to this level. To go above this level, the ECM needs to control the solenoid. This is why when the solenoid fails, it releases control of the actuator (defaults to open on failure) to only the mechanical spring pressure. Thus, in your case because you dropped to a boost level below that regulated by the actuator, I don't think this solenoid was your issue...along with the fact that once it fails, it fails. It wouldn't start working and regulating boost pressure again.
So the big clue as to what I think happened is this:
So, they put my car back together and gave her back.. and since then I've driven her for a few days and she's driving like normal
This sounds to me like the took the car apart to diagnose the issue and when they put it back together, the problem was gone. This would lead me to believe one of your couplers developed a leak and when they put everything back together, they fixed the leak.
If the problem comes back again, go around and check the tightness on all those hose clamps for the charge pipes. Chances are you'll find one not seated or a little loose. You can crank those things down SUPER tight without harming anything. Usually I'll tighten them until you can't tighten them further.