Regarding Adaptive Helm

So, the new tank item, Adaptive Helm, has caught my interest because of its unique effect: "Unique Passive: Taking magic damage from a spell or effect reduces all subsequent magic damage from that same spell or effect by 20% for 3 seconds. " (using the most recent PBE description as of this writing). At first I thought its effect was too niche as SURELY there can't be that many spells that can apply repeated magic damage (boy was I wrong). There are some interactions that I was curious about that I wonder if some clarifications could be had.

1. DoT's- do they refresh the duration of the reduction? Like, does every tick of, say Teemo shrooms reapply the 3 second reduction?
2. Instances- how many of these spell reductions can this item have at a time? If against Swain , for instance, he's got Q, E, and R that apply DoT's, and usually (ideally) all three of them overlap. Does this item reduce ALL of those DoT damage at the same time or just the most recent tick? How about an extreme example of a Nunu with Liandry's , DFT, and Corrupting Potion hitting a target using E. Assuming if DoT's don't get reduced like I imagined from point 1, then how about if multiple magic damage champions were attacking someone holding this item? Say a tank with this item gets hit by a Viktor E (laser with upgrade) and walking through a Ziggs W (minefield). Do ALL of those instances of repeatable magic damage get reduced?
3. Spell Effects- there are a lot of things that do magic damage in this game. Some of them may not come from just one ability. For instance, does on-hit effects such as Kayle E, Kogmaw W, or Wit's End and Nashor's Tooth get reduced by this item? I ask because their "magic damage" comes from auto attacks. How does that interaction work? Speaking of Nashor, I noticed this item doesn't seem to discriminate if the magic damage comes from champions or not. Does this mean Baron (who apparently does deal some magic damage) gets reduced too?
4. Damage Amplifications and True damage- if a champion has a way to amplify damage, does this item reduce that magic damage before or after the amplification? Also regarding the new Amumu (whose passive was changed to make targets take some True damage whenever they get dealt magic damage), how does he (or any of his magic damage teammates) interact with a target holding this item? Does the true damage invalidate some of this item's effect? This topic of true damage also applies to spells such as Ahri Q (which does true damage on the way back).
5. Other random examples: How does this interact with champions who can deal multiple instances of magic damage with a single spell due to how their passive effects work (such as Lux , or Leona )? Champions with spells that deal "pop" damage after a delay (Vladmir Ult, which on cast doesn't do damage but amplifies damage on targets (which btw will this item recognize the "amplified damage" during this ult's duration as part of that spell's effect and thus reduce the "amplified damage"?)).

Another concern I have on this item is this: looking at all the champions who deal repeatable magic damage from a single spell in the span of 3 seconds (whether its a DoT or a spammable spell like Karthus Q), lots of those champions would get shut down pretty hard with this single item. This situation reminds me of what Executioner's Calling has gone through. It's a very specific counter to an effect that some champions are built around. The difference between this and Executioner's, I feel, is that this reduction is tailored to be more general. What precautions are there to prevent this item from running into the same problems as Executioner's? Is the build path and price of Adaptive Helm balanced under the idea that a tank champion could rush this item first and thus shut down lane opponents who rely on the kind of play style this item is tailored towards? Wouldn't this exacerbate the top lane tank meta even more? The effect of this item feels a bit overbearing towards what seems to be a relatively large population of champions (compared to Executioner's, which arguably has a smaller pool of champions it directly affects). What are some of the precautions or counterplays in place for this item?

Tldr; new item's effect affects lotsa champs. does it do this and how is it dealt with?