I picked up a large selection of CSC MARMITTE products quite a few years ago, and was very happy with what came in. And I sold most of it over the years, all without complaint (except for the price, but hey...).
A quick check shows I still have a Lancia Beta header, and an 850 Sport exhaust system. Both of which I had originally ordered more than one of.
About 2 years ago I restocked on a header for a fuel injected 124 Spider in the workshop, and the bung for the O2 sensor was mounted directly to interfere with the frame rail of the car! The picture of this product on my web store is correct...
I had to have the bung relocated, which really wasn't a big deal for my welder, but was very frustrating for me as the installer. I was also unhappy with the finish quality of the flange sealing surface, but I fixed that so it would not fail gaskets. My client was also frustrated because of the additional delay in completing his project, but in the end he was happy with the product.
Positive is it fit great (other than the above), so the main jig was accurate.
CSC MARMITTE never responded to my concerns with their product, and your issues explain why. But they did fill my order almost immediately!
Overall it is (probably) the same old story that is happening with all of the old Italian companies. The old generation that made the company, and continued to make it work for years, are dieing off. The family either does not want the burden of the family business (so they scrap everything or sell it off), or one of the kids/grand kids takes over. Usually the new staff has no real idea what was going on, and the quality control part of the business vaporizes. Of course, there are some positive exceptions when the family actually does better than the old guy! And some of those new owners are going out of their way to make the family dream right.
(I wish I knew one of those kids...)
On the silencer rattle. I purchased (from a different company) ABARTH 1000 OT muffler reproductions, and have sold a lot of them since. But wouldn't you know that the one I pulled out of the pile and put on my own Multipla rattled terribly from an internal baffle. It just so happened that I had installed it just before Concorso Italiano where I had the car on display. The company owner came up to me and introduced himself, and thanked me for the business I was doing with his company. I took him over to my Multipla, showed him I had his muffler installed, which made him smile. So I started it up. He excused himself, and came back with his engineer (who was also at the event), asked me to start it up for him, and the two of them listened (and the engineer got down on his hands and knees).
The smile was gone.
After a quick discussion in very rapid Italian, the owner pulled out his wallet and paid for the muffler on the spot.
Funny thing is the silencer is still installed, and after about 5,000 miles, the rattle went away! A motor later, it still sounds great! It was nothing a bit of oily carbon couldn't fix...
Don't give up on Katia yet!