1. After using F for fullscreen viewing, LR advises to use Shift+F to exit. Upon clicking Shift+F, nothing happens and I cannot escape full screen viewing. Not escape, not space bar, not return....nothing.
This bug has been acknowledged by Adobe in the official Adobe feedback forum: Lightroom 6: Problem when viewing images and video full screen (F key related). Add your vote and opinion to make it more likely Adobe will prioritize a fix. See that thread for how to avoid triggering the bug.
2. The program is unstable. When I click the Develop tab, the cursor spins (like it's thinking) and stays that way until I tire of waiting and click the background. At that point, LR crashes.
Go to Edit > Preferences > Performance and uncheck Use Graphics Processor. Does that help?
Where in the heck did the amount slider go once you apply the adjustment brush?
The Adjustment Brush doesn't have an "amount" slider. Perhaps you are thinking of the Opacity slider for the Spot Removal / Clone / Heal tool?
YES! Turning off that option fixed the random crashes.