It’s looking like you are going to want to hook the volt meter up to your talk lines. I would test just probing the wires going to the handset, if I understand the schematic correctly the talk circuit is separate from the ringer. That being said, you can get a voltmeter that is tolerant of the voltage. In my experience anything under 400v is fine for most voltmeters, they use a resistor divider that have values in the megohms, the current passed across them is minimal.
As far as blinking on ringing, that one is super simple since the ringer is AC, just use an induction type detector to flash an LED on. Think along the lines of those screwdriver looking line voltage testers that light up when you put them next to a live outlet.
Edit: I should clarify what I mean when I say the ringer is on a separate circuit. There is a coil that energizes when the line goes up to ringer voltage that shunts power to the ringer and away from the talk circuit, so measuring the voltage from the handset or speaker should protect you from seeing the ringer voltage.
It would be how the voltmeter would have been wired back then, you can damage the needle or it’s spring by pegging it for too long. Even then though, the only real issue I would forsee would just be the voltmeter spiking all the way to the right while it’s ringing, the chance of any real damage is pretty minimal. You can also check the datasheet on the meter, it should tell you what it can safely handle.