Well people already gave their thoughts and it seems like you already made up your mind but I thought it'll still be good to give a piece or two about what I know.
1. I couldn't agree less it's a yes and no depending on the situation however based on my personal experience I won't reccommend you prioritize it over other skills unless you have extra cp.
2. i personally go with crackshot myself, wis is easy to get once you use healing set, oh and i sometimes switch my staff with my cleric mace and shield which has wis and extra wis procs, once again depending on the situation.
3. i did level it, prioritizing it too as a matter of fact. true the def bonus is not that high however i've been with random parties and that thorn shield can keep your dps alive more, or those mages that steal aggro? Some can kill before the mobs reach them but that's still a chance, I learned that casting thorn shield on them actually helps a bit, less dying rate. Idk, that's me, each have their own preference.
4. Instant cast speed isn't that hard to get once you reach higher levels because there'll be more stuffs with cspd procs, not to mention shaman has agi buffs. Still try bring it up to a decent level I guess.
On a side note, I don't think you need to have like 1,9k or even more wis to heal everyone to full health in kasaadin. Idk, I only have about 1,5k wis when all procs are activated though I have some g heal procs too, but even when my procs are off, I only have about 800 wis because I always heal kasaadin with shaman, 60% mcrit rate, I can still heal everyone to full health, or nearly full. However, based on personal experience, full wis build is more stable on trials and instances as the minimum amount of heal you have should be decent enough.
Another side note about cleric and shaman, well both have their own advantages/disadvantages. I switch between the two (or three, with bard) depending on the situation and if you build your shaman right, you can heal high. My shaman can heal almost as high as my cleric, not counting in case i put on totem of grace so idk.