The Cleaner is a tool that has a special piece of code for each virus, to clean up its registry entries, temporary files, etc. This requires special processing of each virus (more thorough than just adding the detection to avast!; besides it has to distinguish more variants than avast!) - I’m afraid I won’t be able to add all the avast! detected viruses in the following 1000 years (even if no new viruses were added to avast!).
Cleaner doesn’t run in DOS environment, because it’s not a DOS program. In fact, making it a DOS program would require to rewrite most of its code. Besides, you don’t have access to the windows registry from DOS - i.e. fixing the registry entries (which was one of the main reasons to create the Cleaner) would not be possible.