0
Is there a way, in theory, to export private key from the bitcoin address (watch-only) that is now expired, but used once before? (even to get a different format of that key, numeric one instead of QR code used in the first place?). If everything is traceable, out of curiosity - has anyone ever managed to actually do this?
3
It is not possible to compute the private key for an address, ever, unless there are severe bugs in the wallet that holds the key.
Also addresses don't expire. For privacy reasons it is bad to reuse them, but their security does not depend on this.
In addition the "now showing as watch-only" in your question seems confused. Addresses don't become watch-only by being used; if it's watch-only it's because it has always been watch-only.
Well some obviously do expire on certain sites, for security reasons (some change after every week etc.) I understand the question might be confusing as I am also not an expert, I am trying to explain situation that I'm faced with and describing literally what I'm seeing from my end. Thinking that, because of the security measures set on that wallet (they have been changed after certain amount of time), could it also be that they - once imported in another wallet - become watch-only now? – Zion – 2019-07-19T15:33:57.793
@Zion I don't have enough information to say for sure, but my theory is that you have been scammed. Is it possible someone gave you an account, or let you import information in an account, and the told you to deposit coins in it so that there is enough to withdraw? If so, the watch-only wallet was always watch-only. – Pieter Wuille – 2019-07-19T15:53:07.317