Hardware wallets aren't without their flaws. With an application-level vulneravility in a hardware wallet, you are still screwed. Here is just one example: https://www.ledger.com/improving-the-ecosystem-disclosure-of...
Signed transactions cant be modified.
Someone needs to make sure nodes see it that transaction and add it to the database, eventually.
This user experience has not been refined, but is very possible. A system with fewer nodes, like if the internet was attacked and not available, would still work for this currency.
How big is that signature/file? Can it be encoded in a QR code or something simpler to bridge the airgap?
