Usually having a large litter is counter predation, not to adjust for health issues.
Have you tried speaking with someone who does breed them successfully, so see what feedback they offer? I'd actually e-mail three or four people to see if they have any special tricks, or have seen the same thing.
As you don't have specialist vet, is there maybe a local university with a biology department that might be willing to dissect a deceased animal? Even if they aren't experts, anything obvious should stand out, especially if photos are taken during the process. They might even do it for free.
Finally, are there more than one subspecies of this species? Any maybe crossing them does not work? (I realize that's a stretch, just throwing it out there.)
Best of luck to you and the last baby -- again, glad to hear he ate!
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