If they have the ability to take your computer, presumably they have the ability to search your residence. Where would you keep it out if reach while also being able to regularly use it? Or what would you do if the police showed up while using it?
For your first question, it's a stash at a walkable distance out of your residence. For fingerprints, wrap it up in a condom every time you fetch it. If you live in an apartment complex, store it out of your apartment at a place where people would never look thoroughly: the garbage room, for instance.
There should be a way to make the Tails OS in such a way that each time the USB is plugged in, the contents of the flash drive are moved to your computer RAM, so that if you plug it off without running `shutdown` explicitly, not only you'll shut the computer down in 2 seconds, but your USB is also clean. The `shutdown` command being run with the USB still plugged in then copies the image from RAM back to your thumb drive. Your non-destructive shutdown routine will obviously take a lot longer though.
you're over thinking this. The USB does not even necessarily have to be undiscoverable, VeraCrypt is enough. Just put a couple of K on some crypto wallets and hold them in the main volume.
The fact that you've got valuable data on the "boring" volume is enough to explain why it is encrypted.
If you use an amnesic OS that does not right to the USB and is read only on the hidden volume, you won't be racking up the SMART data clocks on write cycles when within the "interesting" volume, making it look completely like a passive USB, sat there looking pretty and gathering dust just holding your wallets.
But even then, you could explain any high usage in the SMART data as it being a random USB you had lying around that you used to use daily, that you decided to throw your wallets on. The SMART data is not timestamped, it's just a total cumulative counter that goes up over the entire life of the device.