I'm not sure that I completely understand your post. In any case, let's see if these comments help to clarify your concerns with the Luna after performing routine battery replacements.
First: If you press both outer buttons at the same time... provided you Luna's sensors are dry... your Luna should power down.
Second: Pressing the outer right button will turn the light on. Keep in mind that the light will remain on for a few seconds. You can adjust duration of light in your settings. Please note that this right outer button also invoques the compass. One more thing, this outer right button, like the accompanying left and center buttons have more than one function, they also help you scroll through the different menus as well as change settings. If you need to go back to the main screen, press both outer buttons at the same time.
Third: Pairing the computer and transmitter requires very methodical steps. You have to follow these steps very carefully. Once they are paired, they generally stay paired.
Here is what you need to do.... step by step... very important.
1. Place your first stage onto a tank. Don't open the valve, don't pressurize the system.
2. Turn your Luna On.
3. Place your Luna, which ofcourse is powered on, immediately adjacent to, and touching the transmitter in the un-pressurized first stage.
4. Keeping them (Luna and transmitter) in contact, open the tank valve and pressurize the first stage. Don't separate your Luna from the transmitter, keep them touching each other. If you are unable to keep them in contact, you will have to start all over again.
5. If you did this correctly, your Luna will say in about 5 seconds... Pairing Tank 1. Go ahead and press the right outer button.
If you don't see a message, purge your second stage a bit, don't go overboard, but some slight purging will work just fine. Remember, keep the body of your Luna touching the transmitter.
6. After they have paired, look for a PSI reading. It should be displayed on the lower left corner.
7. If this failed, start back to step 1... make sure you fully depressurize the regulator.
Here is a link to a video. It's for the Sol, slightly better model than the Luna, but the steps are the same and the menu options discussed in the video will hold and apply with your pairing procedures.
Hope this helps you. If after 3 tries you don't succeeded, you may have to take it to a shop for service or checkup.
Ricardo
PS: These computers are exceptional, reliable and built like tanks. Don't let the pairing procedure discourage you. Once you get it paired, it should stay; however, it does not hurt to pair it before each use.