A full node is capable to handle temporary chain splits and to choose the right sequence by checking for double spend and not the signatures.
That's a nonsensical security model. You're allowing an attacker to take anyone's money with invalid signatures, but not spend it twice? He'll just pay you and then steal the money back. No need for a double spend.
Without the ability to validate signatures, you can effectively not validate any useful property of the system. If that's the model you want, it already exists, and it's called a light client. All SegWit does in this regard is reducing the bandwidth for such nodes. It doesn't change the requirements for nodes that do need to validate.
5- March 2028: The court issues a verdict: "As there is no electronic signature proof 'attached' to the transaction
There is just as much proof in SegWit transactions as in others. The only difference is that it does not contribute to the txid. It's still included in transactions and blocks, it's still required for validation, and it's still committed to.
There is indeed a new storage model possible where someone chooses to delete old signatures but not delete the rest. However (1) there is already no guarantee that nodes keep around everything for you, and if you want proof in the future that a transaction took place you'll need to keep it yourself (which every wallet software does) (2) that new model is only useful for serving light clients that already don't care about the signatures anyway.