I'd have to look at your actual waypoint route to answer definitively, but a reasonable guess us you're simply seeing the fact that the WP algorithm creates a path between waypoints using a bspline curve, so you can't have a 180deg turn in place...
With RC the left stick up/down behavior goes all the way back to that being an actual throttle on the gas-powered fixed-wing model plane. It was natural for "up" to be more power and "down" less. In a real plane the "throttle" control works...
Got a Neo 2 through an ebay seller, took 5 days.
The biggest problem so far is simply finding manuals and such on line in the US -- DJI has blocked US access to these parts of their website, resulting in 404 errors.
They can be found but...
Just to set expectations... Don't count on an SDK for the mini 5P (or mini 4P, mini 3P, Air 3/3S, Mavic 3 series, Mavic 4 series, and I'm sure a few others I've missed) EVER being released.
This has been a sore point in the user community now...