Thaeriel works like other delves, you choose 1 out of 3 cultists three times.
Back when chosen visions was still around, +2/+2 to the chosen one, chosen one was a rogue deck. Either you got a 10/10 frontline ward osiris and rolled some people or it got answered and you fell behind a ton. Some decks can handle it well, others like nature couldn't handle a big early frontline. So +2 health was a problem for some decks and you faced an immovable wall. Chosen one should get a glow but who knows what spaghetti code they have.
Blade is as good as or even better than Demo. You can't jam it in every deck. Some lists are too tight to squeeze in two 5-drops.
Hiding your DD on turn 5 is way too slow, just play CI instead. Maybe some weird magic transform deck with metamorph and reflect ele could work.