A ton of Curse related stuff is completely dysfunctional and needs to be fixed asap

-Decaying hex doesn't function with hex bloom support or blasphemy, the curse is applied and the DoT doesn't proc. Doesn't scale off of global chaos damage increases either.
-Impending doom doesn't trigger when the enemy afflicted by a curse dies or gets hit with hexblast
-Blasphemy can't apply multiple socketed curses even if you have curse limit increases, it just chooses one randomly and only applies one to each enemy in the aoe.
-Heightened curse support increases spirit cost to socketed curses in blasphemy.

This is just what i've found levelling my witch and testing on my other character. I'm sure there's more with how ridiculous some of these oversights are, it takes literally 3 seconds to socket any curse and decaying hex, or two curses into blasphemy. I wanted to make some kind of goofy hexblast build and i'm finding out almost none of the synergies i wanted to try actually function at all.
These skill interactions feel completely unacceptable for something that affects an entire category of skill build. The testing required to find this would be socketing any curse related interaction into a test character and using it on a pack of mobs for 10 seconds

In another few minutes of testing i've found out decaying hex is spread through the contagion skill, probably meaning it's an unlisted DoT debuff and not an added component of the curse itself. I'm almost 100% certain That the issue is a matter of blasphemy/hex bloom not registering that you actually applied the hex to the enemy and telling decaying hex to apply itself to the newly cursed enemies. If this is intended decaying hex needs it's tooltip updated(would be super disappointing if it is intended) to be one of those only use yourself skills.
The info panel itself doesn't list Decaying hex as a seperate debuff it says "chaos damage per second to cursed targets" which leads me to believe this isn't intended

Heightened curse is a cost AND RESERVATION multiplier. It says it in game.

My blasphemy is applying 3 curses on every enemy. Are you hovering the enemy and looking at the buffs/debuffs below their life bar? It will only show 1 curse above their heads.

I agree with the rest of what you said that the chaos spells are horribly lacking in all of variance, complexity, and functionality. Not to mention the ascendancies that would benefit this playstyle are weak and poorly designed.