I thought, in a typical setting, you have a volume pot that dumps the complete signal to ground one way, and 'opens' up and passes signal the other way. The value of the pot would mean how far it 'opens' in other words how much resistance there is on the incoming signal first, and you control with the pot what's left of the signal.
All approximately good, so far. Volume pots are generally set up as voltage dividers. For more conventional language, read this
So, in my mind, having a 250K resistor before a 500K pot, would be the same thing as having no resistor before the pot and using a 250K pot.
You lost me. These two situations don't resemble each other in terms of voltage dividers or variable resistors. Try drawing them.
Also: You can
change the maximum output volume by changing the pot value, but this works because you are actually changing the output load, which means you will also change the tone. (Larger volume pots are louder but also preserve treble better.) If I want to limit volume, I usually just add a resistor in front of the volume pot, but this also changes the output impedance. If you don't want to change the tone, best would be to find the voltage divider resistances when the pot is at the max volume you want, then select a fixed resistor that matches the resistance from lug 2 to lug 3 and put it in front of a volume pot that matches the resistance from lug 1 to lug 2. This might be overkill or not, depending on how picky you are and how much difference in the tone the volume pot value actually makes.