How risky is to de-solder an usb plug if the flash-drive contains important files? It would be relatively safe for someone capable, or the heat is way too risky for the chip/content?

If you're halfway competent, it's almost risk-free.

Also there’s any (MacGyver like) way to quickly access the file or reconnect the plug without a solder while only having access to office supplies?

That's unlikely.

I don't know what your story is, but if a key part of the plot is removing a USB fob's connector to hide files, and quickly establish contact with the old plug to read them, you need to rethink that part of the story because the whole thing sounds very contrived to me.