This example is a somewhat simplified version of what happened during the last game I DMed for a bunch of friends. A party of PCs was trudging through the woods when they happened upon a very small outpost in a clearing containing hostile elves. The rogue goes and checks it out, and sees two hostiles patrolling, one near the northern edge of woods (Elf1), one on a stone platform about 15 ft from the southern edge of the forest (Elf2).
The rogue goes back and relays the information. If the warrior of the party would simply trudge into the middle of the clearing, all enemies would be aware of a threat and the enemy wouldn't be surprised. They want to start combat with an edge, so they formulate a plan that would cause the enemies to be surprised: the rogue will go stealth up to the northern edge of the clearing, wait for Elf1 to get close, and sneak attack him. When that happens (it's a small clearing, so it's within his vision) the warrior in the party, who will attempt to sneak up to the southern edge of the forest, will run for Elf2 and beat him up. They both roll great stealth checks and head to their positions. So far so good.
The rogue waits for Elf1 to get close and attacks with his short sword. Time for combat, and this is where things got confusing, and where a discussion broke out between players:
According to page 189 of the PHB:
1) Determine surprise
Both Elf1 and Elf2 are surprised, because they didn't notice any threat.
2) Establish positions
Should be clear from the description of the situation above.
3) Roll Initiative
The warrior gets a 20, Elf2 a 15, the rogue a 5, and Elf1 a 1.
4) Take Turns
The warrior can go first. However, as the party discussed before, he was supposed wait until he sees the rogue hit Elf1. The rogue's attack is what started the combat encounter, yet the attack hasn't been resolved yet due to his poor initiative roll. And you can't delay your turn in 5th edition.
Option 1: Some people at the table argue that he can't yet run into the clearing because that would pose a clear and noticeable threat and the elves would not be surprised. He should wait for the rogue to hit Elf1 and use the Ready action on his first turn. This option seems to punish the warrior for rolling well on his initiative.
Option 2: As determining surprise occurs before the initiative roll according to page 189 of the PHB, I would argue that the rules support that the warrior should be able to run up to Elf2 and hit him, without breaking surprise. The fact that the attack that caused the surprise hasn't yet been resolved and that this goes somewhat counter against their agreed plan of waiting for the rogue to hit Elf1, makes this option incredibly unintuitive though.
Which of these options is the correct one? Or is there a hidden option 3?