To answer both queries. First the appicon one.
It is possible to overide or change the def appicon for a given program using another program called MCP short for Master Control Program it is cable of controlling various aspects of workbench: assigns, font, requesters and many others as well as app icon change.
To the second query.
A good GUI for LHA is a program called Directory Opus Magellan version 2. It is essentially a workbench replacement program and it can open .lha, .lzx, .zip archives within lister windows using the XAD library system designed for handling multiple archive types.