#trigger conditions

[magicjudge]

As I understand, effects that double death triggers like Teysa Karlov or the new black Dominus that's been spoiled don't interact at all with Minsc & Boo, Timeless Hero's -2 ability, though I don't quite understand why. Is it because it's part of the resolution of an activated ability?

The reflexive trigger contained within M&B's -2 ability doesn't get doubled by Teysa because it's being triggered by a creature dying but rather by a creature being sacrificed. While the creature is dying as a result, that's not what the trigger condition is looking for.

Similarly, Teysa's ability won't double exploit triggers or the like either. It will only double triggers that specifically look for a creature to die or leave the battlefield rather than anything more specific.

[magicjudge]

Say I have Panharmonicon and Simic Ascendancy out and I cast a really big Walking Ballista. My thinking is this would make Ascendancy trigger twice as it was the Ballista entering that triggered it. Is this wrong?

No, it will only trigger once.While entering the battlefield with counters counts as having those counters put on the permanent, it's the placing of counters that causes Simic Ascendancy to trigger here and not Walking Ballista entering the battlefield in general. This means that Panharmonicon doesn't care about the trigger and won't make it trigger twice.

[magicjudge]

Does Teysa Karlov cause Heart piercer Manticores 'when you do' ability to trigger twice?

No, this doesn’t work.

Heart-Piercer Manticore’s reflexive trigger triggers when you sacrifice a creature to its ETB trigger. While that involves a creature dying, the creature dying isn’t what’s causing the ability to trigger and thus Teysa won’t intervene and give you a bonus trigger.