A couple more possibilities on why Honeywell was invitied to join the investigation, beyond the ELT being under suspicion as a primary factor in the fire.
1: If the ELT has non-volatile memory for maintenance purposes the contents might help in creating a time line and conditions of the incident.
2: I took a look at the first few pages of this thread and see no reports on who discovered and reported the fire.
Theres is one qoute re "sparks in AC unit" but with no information on when in the event that was seen.
Begin pure wild speculation, I realize this is unlikely but stranger things have happend:
Is it possible that the fire burned the insulation on the cockpit activation circuit and triggered the ELT?
Did the fire brigade respond to investigate a possible crash?
Even if as is totally likely the above is not the case the ELT may have been triggered so there could be questions on its performance under fire.