If you are used to the clamping force of the DT1350, you will find the M500 very loose.
Regarding to the pivot durability, I found if you rotate it alot. It tends to get loose quite easily. I just dont ever rotate mine. I've been using my DT1350 for about 3 years now. Still like day 1.
What do you mean selling it sideway? They said they got the batch from V-moda. I guess I'll have to ask V Moda about it. It does concern me a bit. Fortuantely I picked up another pair on head-fi which was bought from V-moda.
I got one from them and I also asked them about the warranty. They said they got their order through V-Moda. Your M100 is covered by massdrop for one year. I said on the package it said about 2 years warranty and they replied if it is over one year, you need to contact V-Moda directly.
It still worries me that the SN code are missing on the headphone, box, and little user manual.