There are two forms of ownership discussed here. First, is the ownership of the account itself. It's either owned by "root", or by a reseller account username. This is in no way related to the ownership of the files within the account's home directory. It's to determine which accounts belong to a reseller, and which accounts belong to root. What you are referring to in your posts is the ownership of the files/directories in the account's home directory. The ownership of the files/directories is completely unrelated to the option you are referring to. The only way to change ownership of these files/directories is through the use of the "chown" command.
I also have the issue of not being able to use the drop-down selection menu to change the Account Owner, WHM simply lists options as "root" or "htroot"! Why are all the other account owners missing from this list? How do I do this please?