Your plan is not a bad one, I for one would not tape the key to the safe but that is just my thought. I would create a file within your computer for those things you don't want to carry or put in the safe. We have encrypted copies of SS numbers, passports, proof of insurance and both sides of our CCs, there are a few other things in that file. Then send or deliver the originals to your niece.
I tinker with stuff and one thing I like to do is rekey locks. I have Listed the type of blank and cut numbers for every lock I have in use. I have them coded so it would be very difficult to use the information and that too is in an encrypted file and a copy in my wallet and my DWs purse and one somewhere in the rig. If my keys are lost or stolen while we're traveling, I can go to a locksmith and get new ones made.