These beings agree that those touched by the rain wilds are partial elderlings. most of them are no more than a few percent, though.
The presumption that elderlings of the stage Reyn/Malta is at cannot have viable children comes from the fact that Tarman had to help it remain alive. As for why tarman knew that? Thats fairly simple... Not even close to all the old era elderlings were born that way. thus it would be natural dragon knowledge that elderlings of certain stages cannot produce live children. In early stages (Jerd, DH), the child becomes more serpent than dragon than human.

These beings believe that in fully changed Elderlings, their reproductive systems have changed sufficiently to reflect dragon tendencies. Where humans can (relatively) easily become pregnant, long-lived races do not have quite so easy a task of it. Dragons have to wait over a decade after cocooning before they can lay eggs. Thus it would stand to reason that fully changed Elderlings simply have to work harder at becoming pregnant, but once pregnant and within reach of dragonkind, the children should be viable most of the time.
Before they are fully formed, Elderling bodies are still in transition. Since the changes are on the genetic level, the DNA of a transitioning Elderling/human is unstable. When DNA is unstable, it is hard for the body to replicate it, since one strand might not be the same just a few days later.
One thing 'thul do wonder about is how long the periods of gestation are in Elderlings and dragons. Obviously eggs aren't laid instantly after mating. Considering how suddenly Malta's birthing came upon her and how (relatively) easy it was, the period of gestation is probably shorter than it is for humans. Shorter gestation typically means smaller infants. The potential presence of claws also supports this as a good idea.

Just because Tarman knows what to do doesn''t prove it's based on Malta & Reyn's stage-of-Elderling. It only proves that it has happened before, and Tarman knew what to do.

It is YOU theory that it's because of DNA progression in the parents.

It is MY theory that the issue arose due to the fact the child is not fully human, period. And needs to be touched by a dragon. I believe that child would be changed no matter what "stage" malta & reyn are at in their Elderling-changes.

These beings agree the child would probably be changed no matter what stage its parents is at. They theorize further on possible effects of the change upon reproduction.

Length of gestation will naturally affect childbirth. If a child is still fairly small, it will naturally take its mother less effort to push it out than if it is large.
If you think a bit on how egg-laying creatures develop, you'll realize that much (or most) of the development happens after it has left its mother. Since dragons lay eggs and humans give live birth, Elderlings will most likely be something in between. Obviously a lot of the details here cannot just be looked at through the lens of science, but also have to be filtered with magic. The dragons, humans and elderlings of the Realm of the Elderlings are far more genetic and scientific in nature than is common most fantasy universes. Dragons don't just use magic to fly, but have to develop wings sufficiently to do so.