1 - Does the chance of spawning herald have anything to do with mf percentage?
2 - Is the main allure of hunting heralds and later levels of them, the sunder charm drop? Is that based on magic find?
3 - when you terrorize a whole act, do you .... kill every monster or do you just go around killing the elites?

1. No. There is a preset amount of mobs per map. Though they vary some and you want your tele map to avoid any permanent unique mob (like shenk for example)
2. Latent can drop from any herald but the higher level the herald the better chance as they drop more stuff
3. Best bet is to speed kill the champs and unique mobs. Slow down when you draw ire as there has to be a mob left to spawn the herald in (they don’t spawn on their own they take over a spawn so it has to be somewhere with mobs you haven’t seen- ideally when you get the note you go to a new zone in the terror area to force the pop)

Sunders only scale with magic find up a bit
There's a 1 in ~1300-2000 chance or any herald will drop a sunder grand charm by upgrading a normal grand charm into a unique, which could also be gheeds. That scales with % mf, could become as high as 1 in ~500-800 with 300% mf
There's a 1 in 238 chance that tier 4-5 heralds will drop a sunder, which scales to 1 in 60 in a P7P1 game, and 1 in 30 for a P8P8 game with all 8 players group hugging. That doesn't scale with %mf, just the player/party count.

If you could have 7 followers humping your legs in a TZ game you can expect ~17 heralds from a full clear of all acts and with a hork barb about 1 in 17 heralds result in a latent sunder, an average of one per full game of P8P8 + horking. So even with that insane setup you'll still have some games with zero sunders, maybe some with multiples (only one type of each per game since its unique, but multiple sunders can drop as long as different varieties)

So yeah its shit odds