Some harsh words here!
You can argue about cage size, but I've seen plenty of monitors kept in tiny cages and it certainly doesn't cause them to suddenly die! (No, I'm not advocating small cages, monitors are something I've always used large enclosures for and they definitely enjoy/benefit from as much space as possible, unlike snakes).
I've kept multiple generations of monitors with zero UV, never had a hint of MBD, and I've used various doses of supplements including heavily dosing literally every single feed with calcium, D3 and multivitamin and mineral supplements from hatching to several years of age (this is decades ago now, there was no information available at the time, flame me if you want but I don't much care). I failed to cause any identifiable problems even while experimenting in the dark and using extreme doses, so I'm a bit sceptical that calcium or D3 supplements are particularly dangerous.
I'm not sure what happened to this monitor. I'd perhaps guess that it was off to a bad start with stress and slow feeding and just didn't really recover from it, but without seeing what it looked like over time it's difficult to say. If it was indeed a sudden death and not a long-term poordoer which finally fell over, I don't think any of the issues discussed here are likely to have been relevant. Sudden deaths are usually either from poisoning, injury or congenital issues. Parasites and diseases don't cause sudden deaths in reptiles - their metabolisms are too slow and these issues take a long time to develop and show symptoms along the way. Even in mammals it's very unusual for a disease or parasite to cause sudden death without symptoms over some period of time. Rare exceptions are parasites which suddenly block/interrupt the heart or lungs, but I greatly doubt that would be the case here.
Pictures or a description of what it looked like over the three months you had it would help. Looked fine on arrival. One possibility is that it was a congenital issue which was always going to reveal itself once the animal started to feed and grow. This happens to a small percentage of all animals including humans (often referred to as SIDS etc).
Oh, and as for 'whole prey', there's a big difference between whole vertebrates and whole invertebrates, especially in terms of calcium, but there are several types of whole vertebrates which any monitor can eat from the day they first feed. I used to use a lot of whole fish for my monitors, and a hatchling Heath Monitor can get down mice larger than pinkies.