Antitoxin Spray and Mender's Purity
I guess it will take a Dev to answer the mechanic behind it.
Or just test it out. I don’t have the skill and I don’t intend on getting it, but what you’ve described would only take a couple quick tests to figure out.
For the first just find a buddy on an enemy server in wvw and test it with that. No need for the pollen, you’re not really testing that part of it.
For the second…same thing basically.
If you read the wiki post on Mender’s Purity that you linked it specifically says that conditions are moved after the heal goes off. So if you’re poisoned you still take reduced healing.
While that isn’t conclusive, I highly doubt the skill triggers a seperate power before the entire power completes. Powers like Mender’s Purity or Centaur Runes have to be programmed to trigger after the triggering skill completes so that they don’t trigger before the cast is finished and therefore allow for interrupt exploits. Take Lyssa Runes for example, it it didn’t trigger until after the skill was complete then people would take long casting time elites and interrupt them to use the rune buffs more frequently.
Also, fun fact, these sort of skills trigger based on the key. If you’re in the WvW jumping puzzle and drink from the fountain to turn invisible then your (now blank) heal slot key will still trigger Centaur Rune’s swiftness. It’s extremely helpful for getting through the puzzle invis but being able to easily make jumps.