Can I cook meat on low for half the preset time?
I would put it in on the right temperature and take it out after the time stated in the recipe. If you're worried about whether the food is done or safe to eat, use an instant-read thermometer to take the temperature. You can check the FDA's website for safe cooking temperatures:
https://www.foodsafety.gov/keep/charts/mintemp.html

As to your concern that the simmer point is different with different settings. If it were different enough to affect the situation you're describing above, your crockpot would not be safe to use. There's a minimum time that your food has to get to a safe cooking temperature no matter how long you are cooking your food (I don't know the exact time, but I would guess around 30 mins). Many slow cookers initially cook food at a higher temperature to get it to a safe cooking temp (above 140 degrees Fahrenheit) and then lower the temp to the designated cooking temp.