How can I use stemmer with Search API Solr but show non-stemmed results in the autocomplete form?
This is not a "best practice" answer but it might help you get an acceptable result (at least till you figure out how to solve this):


Enable just the GreekLowerCaseFilterFactory in your schema.xml (not the stemmer).
Use the NGramFilterFactory to get partial results in your keywords. You can find information on how to do this here.