For point A, he's a tank. He is very clearly most effective as a tank. Balance him around that. If he builds heavy damage, he doesn't get to use the passive well. That's fine. For point B, you can't say that tanky Sions will last too long when you can tune it around the assumption that they're building tanky. At that point, it's just numbers tuning. Just force him into a tank role if he wants to use his passive. There is nothing wrong with that, particularly considering that he's already pretty worthless when building damage.
It's worth noting that Riot has actually said that a flat rate is something they're considering if Sion proves underpowered. They noted that his passive is a good place to start because it is currently the least impactful part of his kit.
The tenacity idea probably won't help. Any duration of CC stops him from doing his job almost completely because of the short time he's up. It doesn't really make a difference if I am slowing him for 2 seconds or 1 second. Either way, I've probably prevented him from reaching me. If you want to avoid making him tankier during the passive, then he needs a stronger gapclosing method while he's in the passive. More movement speed would be much more effective.