When looking up a user account you'll probably have to enter it as DOMAIN\Username before it'll return anything.

You can promote your user to have Host credentials manually by going into the Users table and setting IsSuperUser to 1 (or True) for your user account.

If you've got user accounts that are missing information that you would normally see when viewing a user account then, yes, that is the cause of the error when you're trying to view all users. DNN 4.3.2 was before my time as far as using the AD provider and much before my time on the project. The difficult part will be fixing the missing information. One option would be to delete records that are missing information and hoping that it'll be filled the next time they hit the site. Another would be to attempt to manually fix the information (btw, whether they have email configured on the exchange server or not is unrelated. What matters is whether they have an email address in their AD user object). Here's a couple of links that I found in a quick search that might help you out... http://www.dnncreative.com/Tutorials/DNNTutorialsforAdministrators/IntroductiontoSQLQueries/tabid/400/Default.aspx and http://blogs.snapsis.com/CommentView,guid,c9d3082b-d7b1-4dfb-8c9c-67c0e864964d.aspx.

At one point, and I'm sure if it was still an integral part, DNN was going to heavily use the aspnet Membership properties that were first released with .NET 2.0. Don't quote me on this but I think the core team has moved away from the plan but the aspnet tables do still contain some important information and do have some related data in each.

Lastly, I would strongly suggest that you start looking at upgrading your DNN site to DNN 4.8.4 to take advantage of the various security fixes if nothing else. If you decide to go that route I'd really try to test the upgrade throughly on a copy of the website/database.