DIY if you have basic skills and a saw to cut aluminum
These are not coated for outdoor use.
Yes you can cut top and bottom.
For ease of inserting into base and hand rail they give you a round top, if you keep full length. The square edge helps lock into your round hole at bottom to prevent movement with adhesive assistance
Square
1/2"
Yes. It is slightly smaller than the 1/2 inch.
Depends on the type of install. If the install is for new stairs, you could get away without a bezel or shoe if you install the balusters during the staircase construction.
The rectangle in the design is 2-3/4" in width and 20-3/4" in length.
You can go into the store and they will place an order with the vendor. But no, these products are not on the shelf in Home Depot.