The story is very incomplete and very convoluted. But I'll make try a sscenerio based on a few assumptions and you tell me if I'm close or way off.
If I understand correctly, you were driving along and the car started shaking. You kept driving. It was determined by a shop at that time that (1) the radiator had failed, likely causing the shaking as the engine overheated. The radiator was replaced (by the Nissan dealer?). The dealer also said you need a new fuel pump.
You took it to a second shop to save money on the fuel pump, and the shop was unable to confirm that the fuel pump was bad. He sugested that the shaking may have been from antifreeze that he found present in cylinders 2 and 3 of the engine. He suggested that the antifreeze got there because either the headgasket had failed or the cylinder had cracked. He said it would be expensive to correct.
So you called a third shop, gave him the scenerio, and he said the engine probably sustained damage when the raditor broke. He doubted that the fuel pump needs replacing.
These are not three different stories. Your radiator broke, the coolant ran out, the engine overheated until the headgasket blew, and between the blown headgasket and the coolant getting drawn into the cylinder the engine started shaking. So, as a minimum, your headgasket is blown. Worst case (and likely) you have internal damage to the engine due to ingestion of coolant. You might even have a warped head.
You're in for a very expensive bill as a minimum....and if you're lucky. The head will have to be removed. The headgasket will have to be changed, and the head milled if it's warped. Even if all goes well at that stage, you've sustained internal damage thal'll shorten the life of the engine.
Sorry, but I think the dealership just replaced the most obvious problem, the blown radiator, and misdiagnosed the shaking as a weak fuel pump. I think the other two shops are both telling you the same thing in different words.