Spoofing IMEI numbers
Yes, you need root and without root, it's not possible

Xposed module XPrivacyLua is what I use. I have the pro version which IIRC is needed for spoofing. Non pro blocks access. There are many more such Xposed modules, you find in the repo. Play Store has also some apps that claims to do this but I have not tried them because they cannot be as effective as Xposed modules.

What you propose is a cumbersome, may (if you have identical devices) /may not work approach and not worth the effort