1. I've imported my database into a DDT profile.  I'm assuming that from this point on any changes I make to the database schema in my development environment should also be made in the DDT profile.  Is it best to try and keep the 2 insync manually or would it be better to make the changes to the DDT profile, create an installation package and deploy it to my development environment?

2. It appears the DDT can't create Logins at the database server level or Users at the database level or am I missing something.  It appears the only security it handles is for Microfour's Role-based Security tool.  

Greg is right...if you look at your file groups, your files names are static.  One common issue is that a database may be detached from the server, but the mdf file is left on disk, so when SQL Server tries to create the file group you will receive an error (which you experienced).  The other time that you get this is just like Greg mentioned, if you are  trying to deploy the same database to a different name on the same server.  This is when you will want to use the $DbName$ tag as part of the file name.  This is what we do with our medical software which allows us to have a virtually unlimited number of deployments of the same profile to the same server, we just provide a prefix that allows us to determine the instance of that database. 

The Browse service has to be running in order to "see" a SQL Server instance.  If you type in the name directly, and the server is there, then it will work.  The other thing that I have seen get in the way before is a firewall.  But I would imagine that you are dealing with the first scenario.