Great explanation. I'm going to dive more into how this works and play around with this.

chaum

I understand all that.  My point is you cannot get the confirmed utxo without connecting to a node.

ChrisS

bitcoin

Bitcoin watch-only wallet vs hardware wallet for long-term savings

To build a tx and sign you do not need a node.
You are already using a confirmed UTXO.
You only need a node to broadcast an already built and signed tx and that can be any node and wallet app.

For example, I am offline on a laptop using a TailsOS with Sparrow.
I build my own tx and sign it.
Copy that raw tx string and pass it to you, over a txt file or SMS.
You take it and paste it into your OWN wallet, connected to whatever node and broadcast it.
Done.

You cannot steal my sats and you cannot change it, because it contain my encrypted signature. If you change it, any bit of it, is void.