I think the answer to the number of buttons happens to be 3
And they will be in the upper right corner.
I sat it in the car, it is the same height as a scangauge but about two inches narrower, and fits. I pretended to use the buttons, and am of the conclusion that a small cluster of them is better than spreading them around.
upper right corner was the easiest place I could get to them and still see the screen.
3 because that is what fits in the upper right with sufficient space between the buttons, with sitting flush with the lcd and not increasing the profile.
So just need to glue down a block there of the right thickness, solder up the switch leads, with a common ground and three signals, and glue them to the block.
edit: re chording buttons. I was actually looking at those earlier today, that is weird. I think the only chord that people might intuitively know is the soft reset, where you mash everything down.