You can save QR code as a file and initialize as many devices with it as you want.

And by devices I mean proper smart cards personally, not old phones, but whatever floats your boat.

This https://developers.yubico.com/ykneo-oath/Releases installed on a java card (cheaper than a yubikey) is perfectly usable with Yubico Authenticator on a modern smartphone over contactless interface.

It can depend on the vendors implementation, but usually they will provide the key in some readable form for the purposes of backup. If they don't, but do use QR codes, screen grabbing the QR code may work for the purpose.

It is possible however that a vendor can choose to obfuscate the key in an app or through some other method. Technically it's in there somewhere, but may be hard to extract. In the case of this phone however the only way in is to have a key.

For many services you can onboard more than one device. I have one old laptop staying at home that just does that, being used as secondary 2fa device.

It won't help me much if I lose my phone while abroad on holidays but if I am close to home this is no big deal. Most services still allows you to use email or sms as other 2FA anyway.