VEVOR Shelf Bracket Foldable, 24” L 2 Pcs , Heavy Duty Floating Shelf Brackets, Brackets for Shelves, 5mm Thick Matte Black L Shelf Bracket,Steel Shelving Brackets with 300 lbs Load Capacity

Yes it is.

It does not come in white.

No, 2 positions, up or down

No they not made for that purpose.

3 or 4 of them would I think

Mine are supporting a counter 60x20x 2, however I only use it in my laundry room to fold clothes. It is perfect for what I wanted. I really like it.