FWIW: Regular ol' mineral oil from the grocery store is cheap, non-compressible, won't conduct electricity (in the voltage range in question) and relatively frequently used for pressure potting applications to far more extreme depths than a typical dive computer is ever going to see.
I have to say, calling a dive computer "critical life support equipment" is a bit of a stretch, and assuming the skills involved can only be performed by "trained" technicians is equally ludicrous. I've never been trained to change the oil in my car. Figured it out on my own. Wasn't that hard, if you have a basic conceptual understanding of internal combustion engines. Likewise, I was never trained to rebuild my engine, or change my brakes for that matter. Neither of those gave me any trouble either. I'd argue automobile brakes are a far more critical piece of kit than a dive computer is for most divers. My brakes go out, there's extremely high potential for a very exciting situation to result. My dive computer dies, I call the dive and slowly surface, making a safety stop. If I'm in a situation the computer is actually critical (i.e. deco diving, overhead environments, etc) then I'm absolutely going to be traveling with redundancy.
I've had three shops replace batteries on my computer for me (admittedly never the manufacturer) and the unanimous impression I received from all three of those "qualified and trained" services is that I absolutely, unequivocally believe I'm more capable of doing a quality job myself.
To continue with the automobile analogy; having a driver's license doesn't make you a good driver. Being a "trained technician" means nothing to me. It means somebody sat through a class that they paid for. It doesn't mean in any way that they have any aptitude for the work they're now "trained" to do.
Certainly not saying all trained techs are no good, or that there's no value in training. Not at all. But it's absolutely ignorant to suggest being "trained," in and of itself, means absolutely anything. "Qualified" is another story, and I'd argue it's entirely independent of "proper training," and also basically impossible to quantify. From that, it's arguably even more ignorant to argue someone who isn't officially trained isn't capable of the level of skill required to replace a battery.
Should every Joe jump in and tear apart their own computer? Probably not. But if they have a realistic idea of the potential risks, the process and skills required, let's assume they're mature and intelligent enough to make a responsible judgment call as to whether they're qualified to perform the work on their own. I think pretending it's full of black magic that can only be harnessed by elite wizards with appropriate experience and training is frankly pathetic and condescending.
