Hi Katherine,
I can only imagine how much more difficult it is to do things using a screen reader, I admire your perseverance and tenacity, sometimes I wish I could see over your shoulder and guide you a little.
It is not impossible to use windows authentication for SQL server with mojoPortal but it is a lot more complicated, it requires the web process to run as a windows user with permissions to the database or using impersonation of a specific windows user, all of which is much more complicated to configure. Generally in the world of publicly available hosting you will find sql authentication almost always used and really sql authentication is also the easiest thing to do on your local machine.
One thing that occurred to me is if you somehow have added your connection string in more than one place in your user.config file, the one closest to the bottom of the file wins, so if you accidently did that and then you kept modifying the one at the top of the file with new passwords or users it would have no effect and could lead you to think the credentials you are entering are wrong because the one lower in the file is the one being used and it is not the one you are changing. Just a theory, you might review your user.config and see if you have the connection string in there more than once.
Best,
Joe