All of your contacts and calendar reminders seem to be available online, right? Then I would agree with the MS Tech support to go ahead and do a hard reset. This usually does solve a lot of issues related to the W10 update.
To save a number from the call history to your contact list, do the following:
Tap on the little profile icon to the right of the phone number. This brings up the contact page for "Unknown contact".
Tap the Save icon at the bottom.
Choose an existing contact to add the number to. Alternatively, to add a new contact, tap the + icon at the bottom, then enter ...
The contacts are stored locally on your device, however there isn't a "phone only" address book.
Contacts are kept synchronised with your Live account (or any other account that supports SyncML, such as Google, or if you have one, Exchange Server - or anything that can pretend to be one), so you can choose which cloud service you prefer to use to back up ...
Only a hard reset worked.
I was resisting that but it turned out well, I just had to reset a few things, and obviously lost conversations etc. but the re-install of the OS and restore of the backup worked smoothly.
I can now happily add & edit contacts.
You should add your account and then sync your phone. Saved contacts in your people part of your account will be display.
but unfortunately I missed a lot of my contacts.
Microsoft must be warned people before updating to win10 but they did not and so many people lost their contacts list!
In Windows 10 Phone, it is very common to encounter an error like this. As of now, you will need to use alternatives to this.
One way is if that person has text you or called you, click on that number and press the save icon. Then click on add a contact.