Poison Damage over Time not displayed properly in tooltip.

I am running a Gas Arrow build.

Question 1:
When I put gems, that affect the hit with the arrow, the damage tooltip for the hit seems proper. Increases increase, decreases decrease, etc.
However, when I put the Deadly Poison support gem (-25% damage with hits, 75% poison magnitude), the tooltip of the poison cloud decreases.

My question is whether this CAN be intended, because to me it makes no sense.
I understand that the poison initial damage is calculated by the arrow hit, but the point of having increases to poison damage and magnitude is to have that damage be higher. If the DoT tick is not affected by Poison Magnitude modifiers, at least somewhere it should say so.

Is there any chance the displayed damage in the poison cloud tooltip are wrong (not affected by magnitude and/or other poison modifiers) or is this the expected behaviour?

Now you probably understand how hard it is to actually test this on a real target, it actually would require putting a specific gear, testing on a mob to see whether the dot is higher or not, without seeing the numbers.

Question 2:
Is Decompose affected in ANY way by support gems, that modify the damage of Decompose? There's no info or tooltip that suggests whether this works or not. The question arises by the following description: Poisons enemies as though dealing Chaos damage equal to X% of Corpese's max life.

Now that seems pretty deterministic - it's a discrete number value, which is X% of the max life. Is that in ANY way susceptible to modifiers? If I can increase it, I should be able to decrease it as well. And if I'm not able to increase it, then a support gem that reduces the damage (for another benefit) should probably have it's decrease part not working as well.

Question 3:
Is the Herald of Plague main effect (spread strongest poison to nearest enemy) in any way stackable with Contagious Contamination ascendancy talent? (It is literally the same effect).

Any info or input on this matter will be highly appreciated!