Anyday BizTalk 2010 would be better.
Go through these tutorials first. It will clear things about how EDI works in BizTalk world.
You can read more here.
msdn.microsoft.com/…/bb226440.aspx
Answers are below
1. Yes BizTalk supports any Adapter to receive EDI files as EDI Validations/conversions are now handled at Pipeline level.
3. Credentials can be validated inside the Orchestration.
If you want full control over the message then its good to get it into the Orchestration.
You can use Request-Response HTTP Receive Port. So the source will be waiting for a response.