The initiative rolls can be interpreted as a metaphor for other circumstances. So if the orc rolls high, imagine that as him turning his head ina lucky timeframe, just to see the attacker in the corner of his eye. But he is still surprised, so he does not get to act first.

To answer: the action that causes the surprise does count as a part of the round. The initiative is rolled when it is obvious that a combat should start, i.e. when one of the players or the DM decides to attack. Whether you roll it before or after determining surprise is of no consequence. But it is definitely rolled before the first attack (the back-stab in your case).

See e.g. this question. By the rules, it would go like this:

• Two opposing parties meet (Renee meets with the orc sentry) with intention of combat (Renee want to stabby-stabby).
• The DM decides which parties are surprised (the orc is, probably).
• Initiative is rolled. Now there are two possibilities – either Renee beats the orcs initiative or vice versa.

If Renee "goes" first: Renee uses her round to abuse the poor orc. Then the orc's round comes up, he cannot use any action or move on this turn and he couldn't have used any reactions up to the end of it (so for example if Renee moved away after the stab, she would get no attack of oportunity). From now on, combat goes as usual, next turn is Renee's.

If the orc "goes" first: The orc cannot do much on his round, since he is surprised. So his turn ends. From now on he can take reactions and normal actions on his following turns. Renee goes next and she spends her turn with the backstab (if she wants to). If Renee then wanted to move away from the orc, the orc could use the attack of opportunity on him. From now on, the combat goes as usual.

How this affects assassinate: If Renee beats the orc's initiative roll, she gets both the advantage (since she takes his first turn before the orc) and the crit (since the orc is still surprised)

If the orc goes first, Renee gets neither, since the orc is no longer surprised by the time Renee gets to her turn.

The rules as written are ambiguous - and so it's up to the DM's ruling (and designer clarification has gone both ways)

Unfortunately, it's not clear exactly how these rules interact. The troll has an ability which apparently specifies the only circumstances in which a troll dies, and these spells can cause creatures to die instantly; both are exceptions to the normal rules about how things can die, so the principle that "specific beats general" guides us in trying to resolve the conflict, but judging which of the two features is more specific seems subjective and ambiguous.

If you interpret the spell's rules as being more specific than troll regeneration, the troll dies; and since it is dead, regeneration becomes meaningless. If you interpret the troll's regeneration as being more specific than the spell, it precludes the death from happening despite the spell's effect.

Purely RAW ruling, on specific-beats-general principles, I would personally read the troll's ability as more specific than the spell; the spells can, after all, be used on many different kinds of creatures by many different casters, but a troll's regeneration is only ever about trolls, so it is necessarily much more limited in scope and therefore takes precedence over the rules of the spells.

JC says the troll dies

Official D&D 5e rules guru Jeremy Crawford weighed in when Rubiksmoose asked him about this issue on twitter:

Rubiksmoose: Trolls say that they die "only if it starts its turn with 0 hit points and doesn't regenerate". Does that mean they cannot be killed by power word kill/divine word? How about Disintegration?

Jeremy Crawford: If you're affected by the power word kill spell, it doesn't reduce your hit points to 0. It kills you. It thereby bypasses features that rely on you having 0 hit points. The disintegrate spell does reduce hit points, but if it reduces you to 0, you're dusted.

In this case he's just repeated previously given clarification that effects which state creatures are killed or die do not function by reducing the target's HP to 0, they just kill the target directly. This fails to address the actual cause of the ambiguity, so we can't really take it as a clarification of the rules as written.

The response in context implies that Crawford believed that this aspect of the troll's regeneration ability is only meant to stop the troll dying due to normal hit point damage (as a "feature that relies on you having 0 hit points"), not prevent any other effect which reasonably causes death.

And then JC says the troll doesn't die

When asked a very similar question on a later podcast, about whether or not the instant death (massive damage) rule could kill a troll, he suggested that it should not:

Jeremy Crawford: So if we're gonna use the troll as the example, here's what we're told: the troll regains 10 hit points at the start of its turn; if the troll takes acid or fire damage this trait doesn't function at the start of the troll's next turn; the troll dies only if it starts its turn with zero hit points and doesn't regenerate.

So in D&D the specific beats the general, and the massive damage rule is a general rule, and here we have the specific troll. But let's also look at the massive damage rule, because sometimes a general rule in the way we might think it does if we're just going off our memory of it...

[... looks up rules, reminisces about killing player characters ...]

So looking again at the instant death rule, the troll's exception overrides the general rule.

Bart Carroll: So it'll be smushed, but it will reform...

JC: It'll regenerate, yep, and that is part of the horror of the troll.

Instant death by massive damage doesn't kill you by dropping your hit points to zero; it just says you die. Dropping your HP to zero is a necessary precondition for this rule to apply, but the statement that "you die" isn't any more qualified than the effect of Power Word Kill. Coming back to it later, JC seems to have taken a more literal reading of the troll's regeneration ability and ruled that it really does only die if it meets the requirements specified by its regeneration ability.

It seems plausible that if he'd been asked specifically about trolls and Power Word Kill again, he might have ruled differently, depending on whether he thought the spell was more specific than the troll's trait. He does preface the judgement by explaining how loathe he is to make rulings about general hypotheticals, because D&D isn't a coherent rules system, and would probably argue if challenged that this ruling was specifically about instant death and trolls, whereas the previous was about Power Word Kill/Disintegrate and trolls, so they don't conflict at all.