Hello, my first concern is - why did the vehicle continue to have a concern in then first place? Did you have to pay for any repairs, or is there some kind of warranty or extended coverage that applied?
A reputable repair facility - dealership or independent - should be looking beyond the "obvious" failure of turbochargers, and looking more towards what else may have failed in conjunction. Certainly, this is not a coincidence; too many common parts working together. In fact, was an internal engine coolant leak the original cause that took out the turbos in the first place? Hard to know from where I sit, but it's plausible. And, if this is the case - if you are paying out of pocket, this could have influenced a decision on how to move forward in the first place.
Getting to your questions -
The sensors and catalytic converter maybe should, maybe should not have been replaced at the time of the long block. Your information does not share any data that makes me think there was a problem present -- unless failures were present and verified at the time of the long block replacement already. In many cases, it is too difficult to tell the source of concern -- if you have a known engine issue, you must resolve that before going forward with any other component. What if sensors were replaced without need at high expense? Same with the cat?
Indeed, an engine with cylinder issues could allow fuel to pass through to the cat and cause over-temperature and damage to the internal components of the cat. But how would you know this in advance? Maybe could tell, maybe not.
In any case, the entire ordeal feels like a bit of a mess, and I'm sorry to learn you've had to deal with it!
Good luck.
