No, Downgrading without a full file and database backup is pretty much impossible.

As for the IE6 issue, a few posts exists in the forums and gemini that might be able to shed some light on this, but I thought it was fixed in the 5.0.1 release.

I haven't looked into the Map module, because I personally don't use it so I can't provide any help much with that question.

EDIT: DNN 5 and IE6 == bad combination, Related Gemini Issues: WCT-9072, WCT-9143

Hi Enzo, Sorry I didn't get back to you quicker. I had to run out this afternoon. I wouldn't classify it as a "downgrade", more a disaster recovery. Either way, the basic steps I would follow would be

• create a new database
  • restore your sql backup file to the new database.
• create a new folder and IIS site
  • extract the backup archive to the new folder
  • edit the web.config to point to the new database
• modify the permissions on the folder/files
• access the site through an existing portal alias

However, I'd probably follow one of the guides which go into much more detail. Mitchel Sellers has a pretty good set of guides for DotNetNuke, here is a link to the article on Restoring or Moving a DNN Installation (From backup files)

This way you can keep the DNN5 site online an work through the issues you're having plus have a staging site for future upgrades so you don't run into this problem in the future. I also upgrade my sites to a fresh copy of the database, on a fresh IIS application pool, on a brand new portal alias to just make sure every portal instance runs without a problem. Then, if/when a problem does show up, you can just stop the new IIS site, and start the old one. Granted you have to make sure your data doesn't get lost, compare any changes in any modules any other users might of had access too, etc etc etc... It can be quite a headache, but after a few huge migrations of portals with 10k active users, you kinda just do all this in your sleep... 

Hope this Helps