Personally I would not lose any sleep over it.
It sounds like what you are doing right now is what therapists call "doubling down on your anxiety". (Google that! haha)
You are not the first guy to weld up a crack. (search for welded manifold Nissan Titan) Plenty of success stories, and as far as I have read none of them end in: "the repair chipped some ceramic material which made its way up the tube and past my exhaust valve into my combustion chamber causing catastrophic engine failure".
My '08 Armada has 130,000 miles and it is probably only a matter of time before I am in the same boat as you. (I have a feeling I will buy new stock parts so that the truck is only down minimal time and I get fresh catalytic converters)
2008 Armada SE