Engine damage from catalytic converter

That is a very remote change since the cats are so far down the exhaust port.

Not sure about the engine sucking a piece of the cat backwards into the engine, but if it was low on oil before then where did it go? If there is an oil problem, a long drive will only make it worse. What exactly did Nissan say was wrong with the engine?

What did the Nismo Tech recommended-did he changed the oil and look for dilution or whatever.

Sounds a little strange. Hiighly unlikley that something could be sucked backward into the engine, cat can cause excessive backpressure but the flow is still out the exhaust, not backward into the engine. Not a good idea to get oil changed where they suck it out, they never get all the sludge in the bottom of he pan. Some shops don't even change the oil,

You might be able to suck some of the oil out and take it to a lab and get it tested and possibly tell roughly when it was changed last. It's possible it hasn't been changed in a long, long while.

Even if something got into the engine I would probably damage a valve which would mean a valve job or if somehow itt got into a cylinder it might score a cylinder, needing a new engine sounds a little much.

In your description of the engine being damaged-is it: turns over and refuses to start or won't even do anything when you hit the key to start. Is the engine locked-up

I want to know what, they told you what the Cat did to the engine. Give us a description is to what's happening causing it to be dead or whatever.

We need to know this before we can continue-we cannot fix your problem if we don't know what's the actual problem. Is it mechanical/fuel/ignition give us something to work with.

They told you the engine is damage because they found fuel in the crankcase-they hinting on a washed-up cylinders that won't hold compression. It might be the case-Get a compression and a leakdown test to confirm that.

About something crawling back to the engine, even if you put a Marble between the exhaust manifold and the C/Converter it will never make to the exhaust port its a one way direction deal even during the valve overlap it won't go in.

Totally clogged-up catalytic converter will prevent the engine to breath In the automotive theory they call it Volumetric Efficiency-How well the engine breaths. Like us humans we inhale and exhale.

Good LUck