If you pop 2 khalni hearts, getting all mountains. The first will resolve, you will have 4 mountains, and no triggers happen from Valakut, the second then resolves and you will get 2 triggers.
If he has something that allows him to sacrifice for an effect, then yes, he can respond to your ability with his own. In which case, the target won't be there when your ability finishes, so you will not exile the permanent.
The best you could do is
T1: Island, 1 drop blue dude
T2: Island, 2 1 drop blue dudes
T3: Island, Grand Architect, tap 2 of you're dudes, play the Amulet
T4: Drop a fatty
That's 9 nut cards out of a 10 card draw, to get you a T4 fatty. Or you can ramp with Primeval and do the sam thing far more reliably by T5 or so.
Now, if Memnite was blue, that's be another thing.
You didn't need turn 4. You have 4 dudes out on turn 3 (architect can tap himself for the mana ability). So can drop fatty at EOT of opponent's turn
Scion says 'If you do, it becomes a copy of that card'. Since Leyline stops cards from going there, you don't meet the requirement of having the card go to the graveyard, and do not get to copy it.
1) Take a bomb you can play
2) Take a bomb you can't play, but cut them from it.
3) Take a mana source if you are running light.
4) Take a mana source for something they are splashing, thus possibly making something useless.
5) Take utility artifacts
6) Take their best combat tricks.
The list goes on and on. Also, you can take their Dismember
Since exiling is the cost, they will see what the card is going to be, and may respond before the resolution by exiling the next card, as long as they keep it under the restricted size, they are able to "go off", by continueing to do this and continue to just respond with different activated abilities. There is a legacy deck based on this.
The ruling is right there, creatures with First/double strike deal their damage during the first strike damage step, this is to both creatures and players. Then creatures without first strike, but including creatures with double strike deal their damage during normal combat damage step.
A first strike creature that is unblocked (lets say a 5/5 for the sake of this, maybe Baneslayer Angel), will deal it's damage before any other non-first/double strike creatures, and potentially kill you/them before the other creatures ever get to deal their damage.
Cards only stop being Modern legal if they get banned...
False Demise
Fool's Demise
Body Double (kinda)
Dreams of the Dead
Back from the Brink (definitely falls under situational)
You didn't need turn 4. You have 4 dudes out on turn 3 (architect can tap himself for the mana ability). So can drop fatty at EOT of opponent's turn
Joraga treespeaker
Nest Invader
Viridian Corruptor
Vengevine
Acidic slime
Primeval Titan
Pelakka Wurm
Terrastodon
Just in green.
If you add other colors, it gets complicated. Like I am a fan of sacing Primeval for a Novablast Wurm..
1) Take a bomb you can play
2) Take a bomb you can't play, but cut them from it.
3) Take a mana source if you are running light.
4) Take a mana source for something they are splashing, thus possibly making something useless.
5) Take utility artifacts
6) Take their best combat tricks.
The list goes on and on. Also, you can take their Dismember
A first strike creature that is unblocked (lets say a 5/5 for the sake of this, maybe Baneslayer Angel), will deal it's damage before any other non-first/double strike creatures, and potentially kill you/them before the other creatures ever get to deal their damage.