I had similar question and asked from my LH rep.
If you have outbound flight non-flex and inbound full flex, it means:
1. you are fee to make any changes on inbound flight (but not to outbond), depending just on the availibility and possi ble tariff rules as minimum stay.
2. youi can not return the ticket free of charge -- returniogn the ticket rules comes form the most restrictive rule.