Do we have reason to think that Venat still resides in Hydaelyn? I don't think we've been given a lot of evidence that she doesn't. But, then, we haven't been given much evidence of anything at all. The true nature of Hydaelyn is still largely obscured to us, since nearly all the information we've gotten on the topic has been from a biased source (Emet-Selch), and the little bit we got elsewhere hasn't revealed much, either. All we really know is that the Dissenters planned to summon Hydaelyn, the Venat planned to be her heart, and that Hydaelyn was, indeed, eventually summoned.
As for Tempering, that depends on whether Tempering is a necessary and innate aspect of Primals, or whether it's something the Primals' creators baked into the product. There's reason to believe that ability to Temper might have been something the Convocation added on purpose - the world's woes were being caused by uncontrolled Creation magic, after all, so one way to combat the problem would be to create an entity that can impose his own control over individuals. The Ascians included Tempering when they taught the Beast Tribes to Summon, as doing so helped encourage the Tribes to summon again and again, and in general caused a lot of chaos.
If Tempering WAS a deliberate addition, then the Dissenters wouldn't necessarily have to include that aspect when they created their own Primal.
Oh, I've little doubt that's exactly the plan they have for Zenos. Even before Elidibus was revealed as Zodiark's heart, when Zenos expressed an interest in Zodiark I predicted that he'd be merging with him eventually. Whether or not I like Zenos, I recognized ages ago that he was being propped up as our foil, and as the Villian Sue to our Mary Sue, I have no doubt that he will succeed at everything he attempts, up until the final battle (in which he'll finally lose to us. Again. Hopefully it will stick this time.).