@Armand > Sorry to hear of your disappointment with your HDJ-1500's.
I've not seen the twisting issue before - do you often rotate the earcup of the headphones, or is it just expanding / contracting the earcup for use and storage?
The cracks are a sign of stresses due to flexing, this can occur in any plastic and it's likely happening because you have a large head! ;) You're not likely to see it crack any further - I have similar cracks on my HDJ-2000 headband and they haven't grown in length since I noticed them.
As for the discolouration, that is simply due to body chemistry interacting with the rubber. Some users will never have to clean theirs whereas others will have yellowing almost immediately because of the oils in your hair or any styling products you may use. You could try using a Mr. Clean Magic Eraser - I used one to clean the Pioneer show headphones and it does wonders.
Regarding the adapter tip breaking off, that is likely due in part to a manufacturing defect on the plastic isolation ring of the connector and an over-tight jack on your mixer. If you have removed the tip from your mixer, you can safely use a new adapter with your headphones but check for wear regularly in case the mixer is the culprit.