Head-angle is about stability on the downhill. A lower head angle pushes the front tire out forward making it more stable at speed and on the downhill.
Stability while jumping isn’t really affected by head angle. I think your problem is that the Kato probably has a wider handle bar, and the Mission 2 will probably have an even wider handle bar paired with a short stem. So the Mission 2 should feel more twitchy then the Kato if its rocking something like 35-50mm stem and a ~780mm bar.
It affords you more stability and leverage, but if you’ve been ridding on a XC type bars it probably feels twitchy. Basically the K2 lets you be more sloppy on the steering. Again its something you have to just accept and get used to if you want a modern Trail or Enduro bike.
Pre-load on a full suspension just means pushing down into the travel to take advantage of the shock/fork rebound. If you have ever pushed down into your fork/shock in the driveway, you’ve already done a pre-load. If you have proper jumping technique you’ll automatically pre-load anyways, so idk why REI is trying to freak you out with mumbo jumbo. Just push down a bit into the travel just before taking off, you probably do it already anyways.