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1. OLED/AMOLED screen have active pixels that light up in different colors, like miniature light-bulbs. LCD screens have pixels that are opaque and change color, and then there is a backlight shining through all of them. In an LCD screen there are two ways of changing the brightness of the screen. You can make the screen pixels more opaque (that's what the bottom ScreenDim slider does) or you can decrease the intensity of the backlight (that's what the top slider does, and it's the better way to do it, as it improves battery life and helps with contrast) In an OLED-type screen, the only way to change the brightness of the screen is to lower the brightness of the pixels, so in principle the two sliders do more or less the same thing (but it's complicated because of how the screen drivers work--I don't have an OLED screen, so I can't test to see how exactly they work).
2. Yeah, LunarMap HD (paid, higher resolution) and Lite (free) is by me. I got into the dimmer business precisely because of amateur astronomy--even if one turns a screen red (on my Archos 43, I use ChainFire3D for that--but it does need root), one needs to dim the backlight when viewing dim objects or else one will have one's night-vision impaired. (No need to dim anything when viewing the moon. In fact, some people like to turn on their porch lights when looking at the moon through the telescope, so that there is less contrast between the very bright moon in the telescope and the darkness outside.)
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