OK with Hos we have some options to start the expansion, one of them to "branch" off at any suiting point in the main game.

What I had been "missing" is some kind of a statement from CDPR about the possible/logical consequences that could arise if

- you can start HoS while you not have finished the main quest of the main game and then return (after finishing the expansion).

If CDPR put HoS after the end of the main game, then turning back to the main game would mean that the main quest line would be over - even if the player went off to HoS before finishing the main quest.

But - since there is the option to start with HoS/to start HoS at any given point during the main quest, it would be not quite logic to have the main questline ended (Battle at Kaer Morhen done, epilogue over) the moment the player actually starts HoS.

Why wonder?

Well, during playing HoS there is at least one dialog line that points out, that the battle at Kaer Morhen actually has happened already.

That means that the player who started with HoS (premade level 32 Geralt), or
everybody that went to start HoS at any time during the main quest will not be able to return (to do the rest of the main quest) after finishing HoS, right?

Sorry for the long post, keeps bugging me (also if you think of WaB) ...

If you play Hos stone with their "premade" Geralt, you cannot play the normal game.

If you sidetrack off your game to do HoS - you would have to do it late game anyway due to the level requirement - so you would be past the Battle at Kaer Morhen, and probably very close to the end, so I cannot see anything being spoiled.

So, I don't see any huge issues.

I heard a character make a reference to something that occurred in my base game but would not have occurred in everyone's game. So I suspect that there are references peppered throughout that will be triggered only if you have completed a certain quest a certain way. In other words, there is some flexibility built in. You can shoe horn this in anywhere you would reasonably have made it to level 30.

I started HOS after the end game so I have to hand wave a bit since the "what happened to Geralt" postscript does not exactly fit. I just resorting to some interesting head cannon and moved on.

Yes, the rest of the game constantly makes references to stuff that's already happened, so it makes sense that the expansion would too.

As an aside, I met Yen in Oxenfurt (can't remember if she had a good reason for being there, I basically went back to my last save before starting the endgame in the original), and there was a conversation option about my new scar.

But, exactly that is the issue:
to play HoS, you will have to install W3 (main game).
So, if you decide to start of with HoS and only then begin the main game with the Geralt that has been through HoS, it will not work?

If so - well, I doubt that I can see the logic behind that, the option to start off with HoS should not exist at all.

Correct. If you just want to play HoS using the "pre-made" Geralt, the expansion will start you at lvl 32 with a standard loadout of gear applicable to that level - you cannot "transfer" into a main game after you finish playing that method.

I would think most would want to play HoS as an addition to their main game - and so, they have to be at least lvl 30+ which means they are near end game. If you have finished the game, you could load a save where you are at lvl 30+ and play HoS. In this method, HoS doesn't really add anything to your main game, unless you then carry on (from where your save was) and do the end game again.

The preferred (my opinion) method is to play the main game (or start a NG+) after installing HoS and play your main game until you reach lvl 30+ (which will put you near the end). Then you just pick up HoS in the same way as you would any other side quest from the notice board at the 7 cats Inn. After finishing HoS, you then carry on and finish your game. Using this method to play HoS seems to make the most sense, logically.

This is a workaround (your opinion), surely.
But: during the main quest you had been east of Oxenfurt, seen the content that had been installed with the main game.
Then, after installing HoS you whitness the wondrously, magical addition of some more acres of land, that should have been there in the first place - meaning installed with the main game.
Now, in a world where magic leaks out of every pore, the sudden appearance of populated areas might be well within the list of possible things - but, imagine this, please:

W3 is installed, HoS is likewise installed. W3 is started and is played finishing White Orchard, Vizima, Velen. All of it, every part of the main quest, every secondary quest, every treasure hunt, every contract.

After Velen you may go to Skellige or Novigrad, but instead you start HoS with your actual Geralt (hard but possible).
Then you clear the area of HoS from every question mark and any side quest that does not include the main quest inside HoS.

OK, with this Geralt you return to the main game, visit Skellige and Novigrad, keep going from here.
(remember: you can go any time back to do the main quest of HoS)

While this actually works so far, the main quest of W3 will be closed to you.
Why.

Now, this game is too much fun to start discussing why this actually is a bad game design, but the situation as it is leaves a lot to be desired.

Well, it's your game. Play it any way you like.

Just for shits and giggles, I played my level 40 NG+ Geralt (which is like a normal level 20) and started HoS. Picked up the notice and did a ? but did not visit the mansion to start the HoS quest.

I was able to continue on my main NG+ quests without issue after that. It wasn't "closed".

Now, I think most people, after starting HoS at a reasonable level (level 30/60NG+) would play through it until the end, and would then continue on the main quest. I can't see why anyone would want to leave HoS part way through and go back to the main quest - though I can't see why that would not be possible.

So, I don't see what your issue is.