Yes, you need to have zero credits on the machine to access all of the options. You may be able to access the double-up option while there are still credits on there -- I'm not 100% sure though. Can't hurt to try. If not, you must play off or cash out all of the credits in order to access all of the settings in the self-test menus.
I have seen that glitch you described on some PE+ software, where the credit meter on screen turns to "0" when the door is open, but then the correct number of credits is restored once the door is closed. It's a display glitch.
Before going to contact cleaner, just trying rotating the blue volume knob back and forth a number of times to wear off any oxidation on the potentiometer. If that doesn't work, then yes, you could remove the MPU board and try a small amount of contact cleaner. IMPORTANT: you must turn the machine's power OFF when removing or inserting the MPU board or you can cause damage. A ground strap isn't necessary, although it's never a bad idea if you have one handy.
To aid with volume adjustment, use the self-test setup screens to get to the screen that shows either "MUSIC" or "TONES" for the sound setting. Use the jackpot reset keyswitch on the side of the machine to switch between the two options. Every time you switch it to "MUSIC", it will play a short melody -- that's a good time to adjust the volume pot to a desired level. Repeat the keyswitch turn to get it to play another melody to fine-tune your volume setting.