As long as you're applying disadvantages to cancel off advantages I'd likely have no problem with it were I in your game.
Sure. I don't know if it's as formalized as that....generally speaking, all the backgrounds in 5E come with a benefit of some kind, so each player will have some equivalent perk associated with what they've chosen.
But then, I pretty much just use that choice as a starting point for the fiction, and use it to help shape the events and complications that they face. A noble just comes with all manner of connections and obligations and so forth.....so I use that to help inform the challenges the party will face.
I tend to prefer the zero-to-hero arc, particularly as it helps allow for some long-term growth and change during a long campaign.
I tend to prefer a variety of character types and backgrounds and so on. Nothing wrong with a zero to hero type story, but there's no reason to limit everyone to that approach.
Two problems leap to mind.
One, if player A claims lordship of Karsos it denies players B, C and D the option of doing so should they have so desired.
Two, it grants potential advantages (wealth, status, authority) that wouldn't otherwise be present; and while some of that can be cancelled out by political considerations etc., to do so presents a here-and-now headache for the DM which could have been thought out earlier had this fact of nobility been known earlier e.g. at char-gen.
One, so the solution is to simply deny them all that option? I don't really see this as a major concern....each player can come up with something cool, or we can work together to come up with something cool. No one's going to complain "but Billy gets to be a noble, why can't I be one too?" Or if they are, then I think it's more a player issue than a game issue, and they'd likely make similar complaints about class choice and gear and so on at every step of the way.
Two, I think this may be a headache if things in the game are largely predetermined, but is not a concern if the game is more about finding the fiction through play. But even with a heavily GM driven game, I think it's just a matter of considering the situation; a character has just revealed a noble heritage that until now has been hidden. This kind of thing happens all the time in genre fiction. All you have to do is ask questions; why was your heritage hidden? Why reveal it now? What will happen now that you've revealed it?
Where you see a headache, I see a ton of opportunity.
It comes under the aegis of player advocacy for their PC, and looking for an advantage.
Well for one thing if it did interfere with my plans the last thing I'm going to want to do is tell them that!

That said, again if I-as-DM had known earlier about this nobility bit then I could have planned around it and even incorporated it in somewhere else if it made sense in the fiction. (e.g. the PC would likely have been dealt with much differently in some towns previously visited, and approached said visits differently also, had the nobility piece been known up front)
Well, in my original example, it wasn't so much about the advantage as it was about sending a signal to the DM about a kind of play that they found boring.
And even though I recognize that we'll likely never bridge this metagame gap we have, I have to assume that you know how small an advantage the fire versus trolls bit is. It's a bit of info to speed one encounter along, not some kind of campaign modifying revelation.
regarding the second point, again, I think this is about looking at the fiction that's been established. Why were there no such reactions in the previous towns visited? Why has no one treated this character as a noble till now? You answer those questions and the ones I mentioned above, and the fiction emerges through play. Again, this is the kind of "Discovery" for which some are advocating.
It doesn't have to be contradictory. You can incorporate it in, and see how it fits and interacts with what's been established.
Yes, which means let's get the important bits of the background known up front rather than appearing out of nowhere halfway through.
I can't make something important in play if I don't know it exists.
Yes, you can! I mean, I believe you can. Obviously, this may not be something everyone's immediately comfortable with, or even that they may enjoy, of course....but you certainly are capable of it.
The way to achieve this (and how I do it, when I can) is to take the scout's player aside and sort the scouting out beyond the hearing/knowledge of the other players, then leave the scout's player aside while I deal with the rest of 'em.
Sure, something like this....or if the mechanics of the game allow the GM to just "fade to black" before the character's actual fate is determined, or something similar, then you've avoided the metagame concern.
Sometimes yes, other times something might happen during that hour e.g. the main party are forced to move and thus won't be there for the scout to find on her return. Or, if the scout doesn't return after an hour and they really don't know why, for all I know they might say "Let's give her another half-hour"; an outcome much less likely if they-as-players already know she ain't coming back at all.
Okay, here's where I think one of the sticking points with metagaming concerns come up. Because there is a difference between the characters doing something that they'd have no idea they need to do, and the characters choosing to do something perfectly reasonable for them to do. If your group says "it's been an hour, let's go look for the scout" and your DM instinct is to say "well why wouldn't you guys wait another hour?" you need to rethink that. Why would they not do what they said? Why find a reason to shoot it down just because it's possible they could do something else?
This is the DM creating a metagame situation where none actually exists. Why fret over the metagame concerns when there's a game to actually get on with? A PC has died and the rest of the group is going to find that out.....why delay such a moment? Why get in the way?