I had the 28mm f/2.8 V3 for a while. its performance was as good as i could have wanted, and was very well made, as far as I could tell. Its only drawback, for me, was too much intrusion into the viewfinder. Of course, using it with a 28mm accessory finder would have solved that. I believe the version 4 was shorter, with less intrusion, which is a good thing. So it's too bad that it has plastic parts to cause concern. My solution was to get the Summicron ASPH, which is also smaller with less finder intrusion. And no plastic parts, that I know of.
Apparently the concern is that the plastic might break, and Leica may not have a replacement. That was my concern when I learned that my 35mm Canadian Summicron had plastic parts. I sold it and bought the ASPH German lens instead, and I've been happy with it.
So, Bad Example, it seems your lens is a disappointment both for its plastic parts, and because it's not really a Mandler design. Why not return it and buy something that was a Mandler creation? Do you have a 35mm Summicron version 1, 2, or 3? They were designed by Mandler, as was the Pre-ASPH 35mm Summilux.