If you want to block the movement of excess rain water long term, you will need to establish a base down into your ground to create a trench filled with base preferably rocks. (Unless you plan to set the retaining block like a brick wall with mortar as the adhesive I would forget thinking that you could glue them together.) After you dig a trench for your base (Home Depot has many rock options in store to use as the base / trench, I like the white marble chips) I would back the neighbor side of your retaining wall with the same rocks you use for the base. This will slow the movement but of course the excess water will make it around the ends of your retaining wall. Nothing will stop the movement of the water though.
No, You would need to cut one in half with a Masonary Blade or Diamond blade
1/2” x 1”
The weight is under the specifications tab on the items page=22.4 lbs.
HD should be able to answer whether half-size blocks are available. Otherwise can you find a local mason who can saw the blocks in half for you?
I use them around my fire pit. No issues so far.