sorry for being wordy. But this will possibly help me in the future as well.
1) if you look at this video starting at 14:55 it seems that this is not necessary. It is not glued to the screen assembly and doesn't cover any screws that are later removed. The whole disassembly is shown here. You will hear that LE55ONS says "if you need to remove the button flex cable", implying that you don't have to remove it if it's working.
Nokia Lumia 920 Disassembly & Assembly - Screen & Case Replacement - YouTube
at 14:55
More evidence that you can leave it on: If you look carefully at the one sold here on ebay it still has the button flex cable attached (second image top left)
Touch Screen Digitizer LCD Display Assembly for Nokia Lumia 920 Replacement USA | eBay
same here on Amazon.com
Amazon.com: Nokia Lumia 920 Full Front LCD Display + Touch Screen Digitizer + Frame Cover Mobile Phone Repair Part Replacement: Cell Phones & Accessories
This one goes for $81 and the reviews are mostly good.
2. Keep in mind that I have only changed housing on mine to this point. That aside, with prices between $67 and $90 on ebay/Amazon I personally would try this myself first. It really depends on your comfort zone dealing with this. I've also read that even repair places may not always do a great job. At $200 on Amazon you may also want to consider buying a new phone altogether, especially if you have to replace housing plus screen.
Amazon.com: Nokia Lumia 920 AT&T Windows 8 LTE Smartphone 32GB/1GB RAM - YELLOW: Cell Phones & Accessories
3. Yes, you are not always getting the same reliability and service you will get with Amazon.com. However, even when I received something I wasn't happy with I was always able to negotiate with the seller to either get a refund or replacement. Sure, it's a hassle, but often either the only or much cheaper option.
I would also take some of the negative comments such as the ones on Amazon with a grain of salt. Of course your screen will break if you drop it. That doesn't mean what you got is not as tough as the original.
Amazon.com: Generic Original Assembly FULL LCD Display Touch Digitizer Glass Compatible For Nokia LUMIA 920 L920: Cell Phones & Accessories
(last review)
good luck