Background apps quietly spending your allowance

Not necessary: some apps need background access to deliver notifications. Restrict only the heavy ones the usage screen identifies.

Because backups and updates are often scheduled for when the device is idle and charging, which is exactly overnight.

Most notifications arrive through the system's own push channel, which uses very little data and generally keeps working. Content attached to a notification, such as an image preview, may not load until you open the app, and the exact behaviour varies by operating system and device generation. The safer approach is to restrict the heavy apps rather than switch everything off.

They start counting at different moments, round differently, and treat protocol overhead differently. A gap is normal rather than evidence that someone is counting wrongly. Use the provider's figure to judge what is left, and the phone's to identify which app is responsible.

Yes, because it is broader than per-app controls and on many systems it restricts background activity across the board at once. Note that some apps can request an exemption, and behaviour differs between operating systems, so use it alongside per-app settings for the heaviest offenders rather than instead of them.

For mobile data, certainly, since nothing is connected at all, but you will also miss calls and messages during that window. Many people prefer to stay on the accommodation Wi-Fi with mobile data switched off, which protects the allowance just as well while messaging apps keep working.

There is no need. Deleting does solve the problem, but at the cost of local data and a fresh setup when you return. Restricting the app to Wi-Fi achieves the same saving and reverses instantly when you get home.