I just looked for you and didn't see a way to lock it. However, I've had no problems with the orientation flipping when it's not supposed to. The only time it randomly flips is if it's in my lap, I lean forward to grab a drink or a bite of snack and it tips a little bit, and the screen will flip thinking I put it in my other hand. But as soon as I pick it back up it flips back. It works fine for reading when lying sideways in bed -- it'll use whatever orientation was last set. If I want to flip it I just pick it up, tilt it til it does, and then set it sideways again.
There is indeed a landscape mode in the font menu's Page tab - I'd forgotten about it until I started composing this reply. When in landscape mode the orientation sensor will switch between the top of the page being on the side with the page turn buttons and the side with the cover hinge.
The sensor will not rotate the page in 90-degree intervals -- only 180. So either portrait to portrait or landscape to landscape.
An upcoming firmware update will add more font sizes, though when I checked today it wasn't available yet. "In the coming months", I think. This hasn't been a big issue for me personally yet though I think I'll be using a size slightly larger than what I use now but smaller than the next size up on the current firmware.
This video shows the invert-colors option in a few places:
https://www.youtube.com/watch?v=iO7rul8rykI&t=1s
This will not change battery life since eInk only consumes energy when the state of the pixel is changed -- light to dark or dark to light. The LEDs are on regardless. I don't know yet whether I'll use this mode or leave it dark-on-light as it is now. It's something to experiment with.
I hope this helps!