Malicious USB devices are fairly common, and this certainly has the 'right' form factor.
There's a reason 'do not plug in a USB drive you have found in the parking lot' is reiterated in every corp security training.
As for inserting keystrokes, that will be obvious if it enumerates as a keyboard.
You should turn down your paranoia a little more.
This is true, but this also doesn't need to happen at insertion time. An HID keyboard can show up, say, 3 hours after you plug it in.
I miss grsecurity's patch set so much. It had an option to defeat this (deny all USB device enumeration post-boot, i.e. after the kernel executes init).
While I can try and conjecture how those might work, that's not really in my lane.