If it does not run the full length, then the floor will have to be sloped in more than one direction so that the water can drain.

Agreed, the drain doesn't need to run the entire length.

These drains can handle a lot of water, so shorter length ok

I agree that most of the time you don't want linear drains to run the full width of the shower.

You DO want the drain across a whole wall in order to avoid a compound floor slope. Multiple planes is much more difficult to negotiate with a walker or chair. Too many people do not understand the required geometry for proper water flow and waterproofing.