I'm going to support most of this apart from the tob being a dangerous death tbh. There are a few problems at tob which can get you killed without it being your fault at all. Maiden is fine, Bloat if limbs fall when he does the moving/dying animation the limbs are invisible and can catch you off guard completely and kill you/hit really high. Nylo boss is fine for the most part but i feel like you have to switch prayers a lil earlier or you will get hit (I don't know if that's just me though) at Sotetseg if you're chosen and he throws the large red ball when you leave portal it graphically glitches so it's invisible to the player who went through the portal (i see this 1 killing people all the time) another problem is you can be pretty much be full hp and 2 different projectiles hit you at once and you get hit for like 90+ damage and die(if this happens at the same time as the dd ball you can't even see the balls) xarpus is pretty much fine to. Verzik can be the biggest problem sometimes. The amount of deadclicks you get are insane half the time making you have to spam click (this one is very dangerous if the nylo spiders spawn since they can hit up to like 73ish and if 2 hit chances are you can't out heal it) other then that this is a great poll with many great ideas especially the nightmare instance.
Changing ToB to unsafe death this late from release doesn't appeal to me but I haven't ever played a HC and don't plan on it. Any current HC who runs / has run ToB may already have BiS gear. It will only put new HC players at a disadvantage and somewhat discourage them from bothering with it.
As what the others have mentioned, TOB should definitely go through a rework first before considering making it unsafe for hardcores. Other than that, the other suggestions seem fine to me.