Is it at all possible to install Portal 2 purely from retail DVD?

Go into Steam and select Steam -> Backup and Restore Games.... In the Backup and Restore dialog, select the Restore radio box and click next. Then browse to the root of the CD. It should say it found a backup for Portal 2. Click next, and it should install the majority of the game.

Note that I haven't tried this and offer no guarantees that it will work better than what you've tried already.

Found the answer on the Steam site.

• Mac users... it seems to be dead.

• PC users : follow these steps.

  1. Log in to Steam and click on Library.
  2. Right-click on the game, select Delete local content, and confirm.
  3. Insert the first disc into your computer.
  4. Close Steam (Steam > Exit).
  5. Press Windows Key + R to open Run

  6. In the Run window type:

     "C:\Program Files\Steam\Steam.exe" -install E:

     Replace E: with the CD/DVD drive you are installing from if is not correct. Replace C:\Program Files\Steam if your Steam installation is not in the default location.

  7. Press OK. Steam will launch and ask you to sign in if you do not have your password saved. Your installation should continue from the disc.

From : https://support.steampowered.com/kb_article.php?ref=5357-FSQM-0382


This works well ! It installed from the disk up to 93%. Then you have to download the remaining 7% before you can play.
Enjoy !