Will the relay break if the disconnected end voltage is too high
I think it will be OK. On page 3 it has this: -


  1,000 VAC, 50/60 Hz for 1 minute between contacts of same poles,
  between terminals of the same polarity


And if there is any doubt still in your mind then you can wire contacts from the same relay in series. And if there is a doubt about this, each contact can be in parallel with a small non-polarized capacitor such as 1nF - this will equalize the voltages when open circuit.