See, this is the reason i usualy quote only sentences i react on ...
Instead of whole posts.
But I would like to see some comments from other players on the matter. Maybe someones playing style is all about scattering the party around the map.
I do ... especialy for encounters that i expect to be toughter ...
Hag, Githyanki patrol, Dror Ragzlin, Minthara, Kagha, Zevlor, Zarys(Zentharim leader?), Flaming Fist, Boulette, etc. etc.
Usualy my character goes to talk, and my companions are hiding all around, in case something went wrong. I admit that it dont play well for me allways ... for example with mage it was source of many deaths.

And yes i know that is not litteraly "around the map" ... but i dare to presume you get the picture.
First let me say that I highly doubt that anyone would leave their party members so far away not for demonstration or exploit purposes.
You know? I watched the video, I know what and why happened. And that's irrelevant.
Maybe unnecesary harsh reaction ...
But lets say i was kinda confused when you mentioned something, as you now say irrelevant, with word
First in this format. O_o
I'm not talking here about the most clever ways to kill the Hag
Me neither, it was just coincidentally the same encounter ...
I'm talking about the system involved in Swen's attempt, which he praised a bit in the video (I gave the link), and how so far for me and one another person that system was more a source of frustration, than a helpful thing.
And im expresing my agreement with Swen ...
Since i also use this system, i like its benefits and i concider it a helpful thing.
No, you don't know the answer, because you didn't understand the question and should read how "surprise" works.
Well, i have my boubts that any rules will have written specific and unchangeable version of how to handle this specific situation ... but if you insist ... i did.

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this one)
Player characters, who's looking right at their enemies from stealth, cannot be surprised, and they are not surprised.
And i still think your character can be easily surprised ... he expected to sneak uppon the enemy, enemy noticed thim, therefore he is surprised. O_o
Also its still just "you may see it as" reaction ... if you want your character to act in his first turn in this situation, you would quite often need to run back in time. :-/
They just start living at different time pase the moment one character attacks and even if NPCs are surprised player characters will be forced to skip the first turn, because again the game considers them as ones, who joined the fight later.
Agreed.
They can attack from their stealth, ofc, like in your example with the Hag, but then they won't have their opportunity to fully utilise all their points (action, bonus action...). They attack, they are getting flagged as "ones, who joined the fight", as well as "ones, who are slightly late for the fight" and will be allowed to act more only on the next turn.
Well ... technicaly theyr oppourtunity to fully utilise all their points
might be extend beyond that turn they are skiping ...
Of course, if we are talking here about ... dunno, Cleric for example, that can Attack (action) AND Heal (bonus action) in same turn, then yes you are right ... to let him join fight "a bit later" as you corectly call it might be a bad tactical decision ... unless he is far enough to not join battle when he cast (cant remember now if you can cast Aid, Bless and such in stealth, but presume you cant) yet close enough to join it sooner than his buffs will be gone.
Yet i still dont think that is mistake of system. :-/
But if we are talking here about rogue for example ... run around whole battlefield like a rabbit (since you have unlimited movement, as long as you stay hidden, wich alone is incredibly powerfull), apply poisons (with unused bonus action), maybe even defuse some traps that will make combat harder ... and then buffed and prepared joining action with attack ... well, that is undoubtly beneficial.
You technically get the same exact situation if you leave the party around a corner and start the fight with just one character.
Agreed.
If you'll bring the rest of your party, they'll start with skipping one turn. That's more or less fair, they really wasn't there when the fight started.
If we focus on that Hag encounter ...
Its much more unfair for NPC undoubtly.
Since i was able to stealth my rogue on one high ground, druid on second high ground, and mage on third high ground ... while cleric was keeping her occupied by combat ... this advantage i would gladly trade for one turn without blink of an eye ...
Especialy if you also get your attack so all you efectively loose is one bonus action (two is rogue is Tief).

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Also if you do the same while they are talking, you dont even need to bother with stealth ... you can simply walk there, turn off all light sources, pull the switch and "save" Marina before the battle even starts, she would not talk to you unless Hag is dead ofc, but you an also shove the Hag while she is talking ...
Same situation, different location ... when you are talking in Underdark tower with Bernard(i think that was his name), rest of your party can walk, loot, and even fight the others ... Bernard and your talking characters are just in different dimension, ignoring all the world around them.)
In the case 2 I described in OP post I'm talking about player characters, who are sometimes nearly breathing in a neck of an engaged NPC, but they'll still skip the turn, because we can't tell the game that all of our characters are going to engage and should join the turn-based mode simultaneously.
The question here is not where your PC is starting the fight ... but where would it start the fight if he would join the fight "regulary", meaning side by side with others ...
If that is closer than two turns, you made bad tactical decision to let him join later.
If that is farer than two turns, you made right tactical decision to let him join later.
Of course both concidering that you want to trade one bonus action for multiple turns movement.
