Or, for example, what if all your communications and actions are being monitored continuously by state-level actors with cameras and hidden bugs, recording every keystroke? Or what if we are living in a simulation, and the malicious operators can extract data from your brain, or just alter reality at will?
But I think you can provide a "practical" level of trustless. That means that, if your adversary was powerful enough to steal your Bitcoins, then they'd be so powerful that they could destroy you in many other ways. So it's a moot point.
A practical solution to your question could be to take all the precautions you've said above, and then simply gradually increase the amounts you are transacting, using a variety of methods, and see if anything gets stolen.
Call it a bribe or a bug bounty or a tax. It's quite effective, because organizations are comprised of individuals, who are subject to human temptations. At least two federal agents independently stole funds from the Silk Road marketplace while investigating it, and they were imprisoned later