We will go OT, but the topic in interesting, so I'll trow my 2 cents. The Old Stonehorn was frankly broken as hell (especially if mounted by a fully equipped Frostlord). Now, it's "just" a strong behemoth, that's still able to deliver an impressive amount of damage (hey, I suppose it's a Destruction thing). The nerfed defence is still valid agains many dangers that are becoming a common thing in competitive meta: mortal wounds, dedicated assassins (Kurnoth Hunters...), or models/units with abilities specifically designed to go against monsters/heroes (K. Venator...). Yeah, it's much more vulnerable to units with lots of light shooting and hordes... but Stonehorn is fast and usually he can charge pretty quickly those kind of threats, and it's still capable to wipe almost entirely a unit in the first round.
I don't see why you should do that but yeah, I guess it would work with rules as written. @Killer Angel why not? I probably wouldn't do it because it sounds weird to me from a fluff perspective. Also you probably don't want to be "that guy". Some opponents will start arguing and it is probably not worth it.
Because the way it's written: "In your hero phase, this model can plant its great Astrolith and activate its powers. Until your next hero phase, you may not move this model" But the question is moot, even if you can it's totally useless to plant it and teleport it.
I struggle with that part since I am not sure if they mean moving it in the movement phase or move it at all. I tend to agree with your interpretation though.
I agree with you guys too, but at my local store a lot of people are interested in/ getting Seraphon and I could see it becoming an issue where some of the more unfriendly players would be jerks about it is all. Thanks as always!