Nobody's saying it's impossible. Again, as I said, Riker and Troi were eventually able to have a child -- I should know, since I wrote the kid's birth scene and came up with her name. The point is simply that it requires medical assistance to compensate for the genetic and biochemical differences between species.
For instance, Amanda carrying Spock in her womb is fraught with problems because her blood is iron-based while his is copper-based; they'd be mutually toxic to each other.
Think of how many human couples have trouble conceiving on their own, but are able to have children with the assistance of fertility clinics. Just because two individuals can't procreate unassisted, that doesn't mean they can't procreate at all. They just need some help.
I understand that and I understood you the first time when you wrote that you are the author of "Over a Torrent Sea". And so what you say is good for the Novel Verse.
The thing is just Will and Deanna having trouble conceiving doesn't seem quite to fit in context with the Series Verse, considering that Deanna was already 50% human and that the other family went through the process five times. Why would they, if they had such trouble conceiving unassisted? Usually when couples have trouble conceiving they go through the process successfully once or twice, not five times.
And why would Deanna have been genetically bonded to a human if they would have trouble creating offspring unassisted? (not that anything else about Haven made sense, but still....)
And would Deanna also have the same problems conceiving with a Betazoid male?
You give no indication at the type of assistance Will and Deanna needed, was it just fertility meds or a treatment that would prevent Deanna's body from rejecting the fertilized ovum/fetus? If so did she need them for the whole duration of the pregnancy or just initially? If the treatment was very elaborate and potentially harmful like the example with Spock, it again makes it weird that Devioni Rai's mother would go through with it five (+ considering that a more extensive procedure had a higher chance of failing) times.
Plus Betazoid/Human hybrids seem to be more common than Vulcan/Human ones (even factoring in Romulan ones)
Again I'm not doubting your authority as far as the Novel Verse goes, but I consider it to be its own thing, not necessarily 100% in synch with the Series Verse and what's written there does not necessarily impact the Series Verse, as far as I understand, just as the Abrams verse is its own thing and indeed one series within the Novel Verse can be its own thing from the rest of it. Different canons.
I can see that making them have trouble conceiving made for the more story possibilities, but making it due to Deanna's 1/2 Betazoid DNA doesn't necessarily mesh with some snippets we got from the show.
Of course I'm also not saying that a line of dialog in one episode should stand in the way of a potentially good story a new author wants to tell. I'm just saying that I was talking about the Betazoids as shown on the series.
Of course the two situations could be easily reconciled by saying that either Deanna or Will had problems conceiving/siring period, without bringing their species into it. Or that Rai's mother simply had a mix of Betazoid/Human genes that made it easier for her to conceive with his father.