[Odsh]

We're running in circles, ain't we :-)

As in all situations in SR6 (not limited to stealth nor if you preroll or reroll), if either side can argue that they have some sort of circumstantial tactical advantage over the other then they might earn a point of Edge for that.

Yes sir, I agree!

Except for the "nor if you preroll or reroll", but more on that below.

And so I'll argue that I have a circumstantial tactical advantage over the other when I'm under the effect of an Invisibility or Silence spell that he hasn't resisted.

And thus...

Grant Edge to the Infiltrator if they're invisible, silenced and such.  Possibly 2 Edge if multiple magical or technological sneaking effects are on.  Let the player decide, use the Edge to improve the Stealth checks or carry the Edge over into combat to represent getting the drop on the opposition.

That's the rub... you don't KNOW if the spells are in effect for the interaction until after the observer rolls.

So maybe the best thing to do is a sort of delayed effect.  Invisible has its threshold.  Observer rolls against it.  If successful, not only is it ignored, the observer gets awarded Edge.  THEN, the infiltrator rolls Stealth against a threshold the Observer just established.  With Edge if the Observer failed to pierce invisibility.

But this have nothing to do with if you roll stealth before the infiltration and then roll perception against a threshold or if you reroll stealth for each single guard. If either side have an advantage (for whatever reason, you listed several) then you can give that side a point of edge in that specific situation. No matter if stealth was rolled as a threshold before the infiltration or not.

Au contraire, mon frère. Not getting your edge until well after the roll was made means you can't use that edge to reroll dice in that roll.  Granted it's usually more beneficial to reroll opposing hits rather than your own failures... but if you're rolling hits to establish a threshold that will then be tested against multiple times, then THAT changes the calculus. Even if you don't agree that nothing beats pushing your hits up higher in this sort of case, giving retroactive edge later on is still taking this perfectly legal option away from the ninja.

[Xenon]

I'll argue that I have a circumstantial tactical advantage over the other when I'm under the effect of an Invisibility or Silence spell that he hasn't resisted.

If your GM rule that you get a point of Edge when sneaking in situations where you cannot be seen (because you are sneaking up from behind or because perception test didn't beat your invisibility threshold or whatever) or if you can not be heard (because you are not within hearing range or because perception test didn't beat your silence threshold or whatever) then GM will just award you a point of Edge (or even two). It is not more complicated than that.

And just because stealth is typically rolled once when you sneak into a research facility (according to SR6 p. 97) does not mean you can't spend Edge on rerolling perception hits for your opponent ('Reroll one dice' is a post Edge action, which mean you take this action after the perception test is already made). But as long as the perception test didn't beat your stealth threshold you can also save your tactical advantage until later, when you really need it.