I think a bag of cement would go a very long way for making mortar for a rock wall. Why make bricks when you can just use rocks and stick them together with a little bit of cement.

Bricks were made for uniformity so you could have it as a known quantity and spit them out of an assembly line. If you're making bricks, you've already lost this game.

Very favorably. Think of how big a bag of cement is. Using a timber mold like the one used in the video you can leave 100s (if not 1000s) of bricks to dry in one day. You need some materials like clay soil, straw and sand, some which might be sourced on-site. It might take a day or two in hot sun for the bricks to dry, but this can be done in large batches with enough space. They also can be fired in a kiln for extra strength, but that would forgo some CO2 benefits.

25kg of cement is roughly 12L of 'concrete. 12L is 20cmx30x20cm (8x12x8 inches for the freedom loving folks)