I think you have been misinterpreting this rule my friend.
"Hit: 16 (4d6+2) piercing damage plus 24 (7d6) necrotic damage, and the target's hit point maximum is reduced by an amount equal to the necrotic damage taken."
Let's say that a given creature has a special ability that does "the target's hit point maximum is reduced by 20" and that special ability does no normal hit point damage.
If a PC currently has 40 points out of 80, the target is now 40 points out of 60, only hit point maximum is affected.
If a PC currently has 60 points out of 80, the target is now 60 points out of 60, only hit point maximum is affected.
If a PC currently has 80 points out of 80, the target is now 60 points out of 60, both current and hit point maximum is affected.
If a PC currently has 70 points out of 80, the target is now 60 points out of 60, both current and hit point maximum is affected.
From the last one here, one can see that one can only modify current hit points for this ability if and only if maximum goes below current. If maximum is equal to or greater than current, current is not affected. The special ability does not state "lower both current and maximum by 20". It states "lower maximum by 20".
The ability here never touches current hit points. It only touches max hit points. But obviously, because current can never be higher than maximum, current has to be lowered if maximum goes below current. But this special ability maximum damage is not applied to both. Just maximum.
Now, you yourself claimed that this all damage is simultaneous.
So let's add in the other two types of damage. Let's use the Chasme damage itself of 16 piercing, 24 necrotic, and 24 necrotic vs. max.
The PC has 70 current hit points out of 80.
Current: 70 goes to 30. (lowered by 16 piercing and 24 necrotic)
Maximum: 80 goes to 56. (lowered by 24 necrotic)
The order here is irrelevant since it is simultaneous. I could have written:
Maximum: 80 goes to 56. (lowered by 24 necrotic)
Current: 70 goes to 30. (lowered by 16 piercing and 24 necrotic)
After all damage is done and the effect is over, then we ask the question is maximum less than current, if so, lower current to maximum.
There really is no other way to do this.
Here is what you might be doing (or alternatively, you are double dipping damage):
1) Do maximum damage first:
Maximum: 80 goes to 56.
2) Is maximum less than current: yes.
Current: 70 goes to 56.
3) Do current damage:
Current: 56 goes to 16.
You are putting an order on the operations (assuming that you are doing it this way). But as you yourself stated, they are simultaneous. Current is lowered and maximum is lowered. Together. The special rule of "if max is lower than current, lower current to meet max" cannot be done until after all damage is calculated. One cannot calculate max before current and apply the lowering rule, or current before max and then apply the lowering rule. The lowering rule has to be last because it only applies after all damage is done (effectively after damage is done, we are asking the question is max still max, if not, lower current).
Now the rest of the ability states "If this effect reduces a creature's hit point maximum to 0, the creature dies". Again, this has nothing to do with current hit points. It's a simple statement that if (in our example above of an 80 hit point PC) that PC gets hit 4 times with our our original example 20 hit point maximum damaging creature, the PC dies.
This is good news for your PCs. Creatures like Chasme and Wights (which I sent at my PCs last weekend) do not double dip damage.