Scenario 1: A ruler mode, if you enable this, you get a single assistant that all other tools will use. Think of it a drawer from which you pick a ruler/ or curve guide. It creates only one type. Just like Sketchbook’s ruler, ellipse and perspective tools. They can get a toolbar button each to enable/ disable them. If you activate one, any of the others is disabled.
If you want to create a more sophisticated setup, you use the assistant tool instead. So two tools, one for simplicity (easy to use activate and forget) and one for complexity (harder to set up, but a huge time saver in the long run).
Scenario 2: press a hotkey and Krita will remap the stroke to a different direction (this only works as long as the mouse button is pressed/ pen is on the tablet. Might be possible to do this for the last stroke as well, when the mouse button is no longer pressed/ the pen leaves the tablet. It’ll just toggle direction for the stroke on each hotkey press.
As a workaround for 1 (not a replacement): there could be a predictive stroke mode, which is a post-processed brush smoothing. This is usually done for vector brushes, but also works for pixel brushes as you can see here in the Sketchbook documentation: Steady and Predictive Stroke - Sketchbook.