Longstem OrganizersLongstem Organizers Over the Door, Wall Jewelry Organizer in Black #6100, Holds Over 300 Pieces

Q: Can you see the door mount on the other side of the door when it's hung?

Yes. It's like any other over-the-door hanging item.

Q: Can you hang on the wall instead of over the door?

Yes, the organizer comes with wall brackets also.

Q: What are the measurements of the door hook? My door is pretty thick.

It fits standard door measurement. Mine is 1 3/8.

Q: Can the hardware be purchased separately?

Everything you need to mount it over a door comes with the jewelry organizer, so there's no other hardware needed. I did mount some felt rounds to keep the organizer from rubbing on the door, but that's all. If you want to mount it anywhere other than over a door, I suppose some other hardware would be needed. I've purchased 3 of these organizers, one for myself & 2 for gifts. They're wonderful!