Dani, I think that you are totally wrong in this case. I have to agree with ManaAdaSystem. A cargo fire, or any fire for that matter is one of the worst things that can happen in an aircraft. If it is serious enough to declare Mayday, it is serious enough to get your ass on the ground as soon as possible, or even faster. After reading the transcript of the SR 111 accident, one of the very powerful lessons was how the crew was dealing with what seemed a very minor technical problem that very suddenly turned into a major crisis. You simply can't wait for some one in Operations to 'advise'. Dumping the aircraft into Chennai costs time, fuel, and perhaps embarrassment on the part of the crew if it turns out to be unnecessary. Compare those costs if you will with the cost to the SR 111 crew. Without diminishing for any moment the very real costs to the company involved in an emergency diversion, they simply pale into insignificance.
I have had the very unpleasant experience of cockpit smoke, resulting in a Mayday, and diving into Haneda in the middle of the night. 45 tonnes of fuel got dumped in the process, and we took a delay of 18 hours before getting to destination. As it happened, the diversion was 'unnecessary', as the smoke came from a fried rheostat which posed no fire threat. Upon return to the head office, the discussion with the 'powers that be' revolved around what I could have done differently that would have resulted in getting my craft on the ground sooner than the 18 minutes that it took me from FL330.
My personal opinion, and that of my employer is that fire indication is to be taken very seriously, and never delay the decision to get onto the ground due to any operational 'cost' factor. Without knowing the B777 checklist, I know that on my aircraft a fire indication means Land As Soon As Possible. There is no reference to 'ask Operations for guidance'. They called a Mayday, therefore I presume that they believed the threat was real. If hindsight reveals that there was no threat, then you are only out money and a red face. No big deal. Anyone who has flown professionally has had ample experience being embarrassed, and the money is the company's, not mine,and not yours.