Can just second Gemma2, 60min after setting the new sensor you just start reading. We observed that during the first couple of hours deviation is somewhat bigger from finger pricking BG values but after 24 hours the match is pretty good, often close to identical. That's why seasoned Libre users often set the new sensor one day before the old one expires. My daughter has been wearing Libre since three months and we are satisfied. As to your second question, you need a Bluetooth transmitter placed atop of the Libre sensor (e.g. Blucon NightRirder) to periodically submit BG reading onto a Android tablet or Phone device running the xDrip+ applet that you can set up to alarm you when BG data approaching preset low/high values while you sleep.
Good luck to stay within range!