In situations 2, 3, and 4 you cannot simply use an existing domain for the output column.
You should either add items to the existing domain, or you should create a new
domain, depending on the THEN and ELSE parts of the IFF function. In case 2 you
can add an extra name to the existing domain. In case 3 you can merge the two
domains, either by adding the items of one domain to the other or by making a new
domain which contains the items of both. In case 4 you can make a new domain
which contains both names.