As for Trisha, it's not so much of a mystery since she was one of the owners of the hotel. Trish and Ash's parents lived in Sunshine Bay, so chances are she lived there too. Also, if I'm not mistaken, shorkake said she never left here, and we know she doesn't know who her father is. If we take into account the mysterious envelope, well... There are only a few pieces left in this puzzle to have the complete picture... don't you think?
Well Lucas... Ahhh Lucas! The god of assholes. He could have been in the wrecker's place during Melody's concert. and for the small town, Sunshine Bay is also a small town and he's here so I think he would have been perfectly fit in this role.