Probably good to have an alert bracelet, but if you were undergoing emergency surgery, the anesthetist would monitor close enough to know something is off and respond appropriately.
Pretty much. Naltrexone is a competitive antagonist against the opioid reception (u being the important one).
They would give the standard amount of opioid (say fentanyl) and if they don't see the desired response they'll up it. Eventually you can "out compete" the naltrexone for the opioid receptor and get an effect.
If they do give 2-3x the dose, they'll be sure to monitor closely after to make sure everything goes well.