I just purchased a cellular iPad mini but I cant seem to find out if charging an iPad mini while using the device affects battery life, and does charging overnight affect battery life?
You should be fine using the iPad while it is charging. The thing you want to avoid that affects battery life is always keeping it plugged in. You want to make sure you regularly "use" your battery, allowing it to drain and then recharging to full. If it is always plugged in and sitting at full you'll start to notice a decrease in performance when the battery finally does get drained.