No, you must activate mobile data and roaming. No connection is possible in flight mode.
No, you can leave it in the device and specify which SIM card is active in the settings.
This is only possible if you choose a data package for several countries (regional/global). Country-specific packages are only valid in the selected country.
Yes, each package has a specific validity period (e.g. number of days) which starts from the date of activation.
Unused data expires after the expiry date and cannot be transferred to other packages.