I don't know anything about tamping paper documents, only that it's difficult in an election when everyone is watching and that we can't watch computer bits.

Offline is certainly more secure than online but software is almost guaranteed to have bugs. An attack is potentially as simple as plugging in an USB stick into the right device anywhere in the chain of creating, storing and fetching the data to view the contents.

The convenience of a digital will may be overall more worthwhile than any security advantages paper has. I fear governments may require users to submit the will using their own proprietary 'black-box' software.