Clogged converter ruined engine?

I have a o4 6 cyl Malibu that was having no issues other than worsening gas mileage the past year, we changed oil with synthetic oil every 3-4000 miles, it was never hot, ran smooth and no engine warning lights. Friday it turned over to 120000 miles and 20 miles later at stop sign I heard it making horrible rattling sound, and the check engine light came on. Drove two miles to my small town mech. He called today and said my cat converter was clogged and cyl 4&6 are bad, getting oil blowback and I need a new engine. Mech says he's never seen this happen in his 32 years working on GM and has talked to others who also are puzzled. Mech can't tell whether cylinders were going bad and that's what blew the converter or if clogged converter blew the cylinders. He also said either way I should have been getting lots of check engine lights and poor or sluggish engine performance, but i never got any lights until the rattle friday and and it ran great until Friday. My husband just changed the oil 3 weeks ago and everything was fine. Right now the car is running ok, a little rough at low idles, but spitting that oil out, which it wasn't before. Anyone else ever had a converter cause engine failure, or sudden engine fail with no warning?

A clogged cat with cause misfires, noise, heat, and choke the engine out.

See http://www.automotiveforums.com/vbulletin/showthread.php?t=972629

I would consider a 2nd opinon from a muffler shop, and have the PCV valve replaced,and a good tune up.

We had the shop do a compression test and leak down test, 2 & 4 cyl will not hold pressure during leak down test and the compression test was low, probably caused by converter clogged and overheated cal. PVC valve was also checked and it is working fine. The mech said 2&4 cyl have blow by that is causing oil to get into throttle body and air filter. Mech cleaned out the cat so the check engine light is off again, but the blow by is still happening.

I've had 2 clogged cats in my time and eack time exhaust pipe flange's got blown out. Didnt hurt the motor of the chevy, but it did ruin the motor in the RX-7 but the rx motor had high miles and it is a rotary engine not a piston engine.