From memory, I don’t believe I’ve ever observed the creep moving faster or slower between Abby and Zagara.
As you already indicated, Malignant Tumor bonuses do apply to creep between Kerri and zerg COs.
Although I am confused on why you think the spread is faster because there is no talent bonus that improves them for Abby or Zagara? And Toxic Nest is entirely different than Creep Tumor, with the only shared attribute being both generate creep.
The only instance I know of for the tumor’s change is Kerri P1, where it’s sight range is increased by +3 to 13, whereas its spread range increased by +6 to 16. The spread rate of the creep itself is decreased to 1/3 to 0.15sec versus 1/2 to 0.225sec in Malignant Creep versus normal of 0.45sec. And the bonus to units on creep goes from a plain 30% movement bonus to +30% attack speed and +1regen to +60% attack speed and +2regen respectively.
Now if you put Kerri and another zerg CO together. All creep will have Kerri’s creep bonus (be it P1 or non-P1). I would hazard a very educated guess that Creep Tumor won’t have the +3 sight and +6 spread (ie. Kerri + Zagara) because it is not a shared structure like Nydus. Think of it as “each CO has a set of unique buffs to various units and structures”, aka talents. As such, it makes little sense to have Kerri’s talent buff to be applied to Zagara/Abby’s unit/structure. However, in the instances of shared structure, we observe this. As such, we see it in “Creep” and “Nydus”.