
As far as the Targ deformed babies, I'm not sure we can be convinced that either baby was truly as deformed as we are told. Both tPatQ and tRP are written from flawed perspectives, years after the fact and secondhand. I wouldn't be surprised if Visenya was simply still born, or born with a heart defect, and the business of the escapes and tail were added on telephone style. It happened with Tyrion, after all.
It is also worth pointing out that there are real life birth defects and handicaps that can cause what we told for Visenya. Look up Harlequin's disease for more info, but be warned that the information is very sad and depressing.
I also don't know if we can take the word of MMD at face value regarding Dany's child. No POV character saw Rhaego's body. Even if MMD told it true, surely it would be a result of her blood magic. Life for Life, Death for Death etc.
That's all not to say I disagree with you that Targs may have some magical draconic blood, I am almost certain they do.