IPhone – Does restoring a local backup using iTunes enable activation lock

This is most likely the carrier intentionally deactivating the old SIM.

When iPhones first came out, your current phone would normally get deactivated at the moment you purchased a new phone and told the carriers to move the service to the new phone. Now, they wait until the new phone checks in to perform the swap to account for delivery delays, backups, etc...

Your first step should be to contact the carrier and explain that you want service on both phones and arrange for which number rings to which device. They might be able to accommodate this over the phone, but if the old SIM has been deactivated in the networks, most carriers are very unwilling (or unable) to re-activate the old SIM and instead ask you to get a new SIM from them at a convenient local retail spot or by them shipping you new SIM card(s).

Apple's restore process and activation of the device doesn't affect network activation and for locked phones, you just need a SIM for the correct carrier to activate the device. iOS does not check or care that the contract is expired, not established or live for iOS activation - just that the SIM is manufactured for the correct carrier.

Following some trial and error, and additional reasoning, I can partially answer my own question:

(1) The events from a local calendar, on an iPhone ("On This Phone", I think it is referred to), are not backed up to the calendar-specfic area in iCloud. Here:

iCloud.com > Settings > (Advanced section) > Restore Calendars and Reminders

Although it doesn't say so explicitly, it makes sense that these iCloud calendar backups cover a user's iCloud calendars. (Plus, my attempt to restore such backups with a hope to see pre-merged, local calendars, failed).

Screen shot from iCloud.com:

(2) The iPhone backup (the complete backup of the whole phone), which in my case was being backed up to iCloud (rather than iTunes), would, I presume, include the local calendars and their events.

However, by time I'd got to looking at this, my oldest 'complete' iCloud backup was just a day old... which was after I merged the local calendars into iCloud calendars.