"Another question about step 5 above: What do I do if I want to utilize multiple compute nodes which do not have the same username and password? Is there a way to specify multiple username-password pairs, or do I need my network to be set up such that all the username-password pairs are the same for all compute nodes?"
You can't do it that way. It's a requirement from MPI to have same credentials on all solving machines.
"A problem I am still having is that when I try to add local host to the list of compute nodes, ANSYS goes back to hanging on "...Determining memory availability on distributed machines...". Do I need to change something about my setup in order to have local host be one of multiple solving machines?"
Let's make sure that in your AEDT solving machines list, you only list two (local host and another one) for now. Then, try MPI test with this pair and with this order. It seems your test above used localhost as second machine and seems fine. So, you could try specifying the localhost as the second in your AEDT solving machines list and try again.