How do Natsu and Happy get into Lucys house?
What you have heard is not a rumour, this reason is mentioned in Fairy Tail manga volume 6 Q&amp;A. 


  Mirajane has an extra copy of her house key which she gives to the other members so that they can enter Lucy's house.


I don't have a copy of this volume now so I cannot confirm it myself, but it has been posted on the Fairy Tail wiki, and a number of other sites.