I started with a USA account, I made a UK account with a different phone number and then after fully verified I merged the accounts. But I cannot remove the phone number from the UK account and replace with the phone number from the USA account because it says phone number already in use.

How can I get "merged" accounts to use the same phone number?

Phone numbers cost money, for each one. I do not need these phone numbers for any reason beyond Amazon requiring me to have different phone numbers, on the off chance I ever need to login to those direct individual marketplaces.

FYI... if this cannot be done then it seems "merged account" is the wrong term here. It seems nothing is actually "merged", but rather it's a single login switcher.