I just say that the only reason your group is traveling together ... is the fact that they all have Tadpole in their heads...
I realize now that I misread your original post. Normally one would say "is to not mention it at all", however you said "is not mention it at all". Without the preposition, my brain assumed you meant "is not mentioned at all ..." Big difference. As I read the rest of your post, I was expecting a solution but only got a list of facts. Hence my confusion

Now that I realize you meant "to not mention it at all", I realize what you were trying to say.
After making this post, I did a bit more research.
This video at 3:58 shows a snippet of text from page 12 of The Illithiad by Bruce R. Cordell (WOC 1998) that seems to indicate the homebrew rule I thought I was making up to explain things is in fact canon. (spelled out below):
"...Generally speaking, the point of no return (in the case of the victim) occurs one hour after implantation . Any time after this, it is impossible to restore the victim; his or her spirit seeks its fate in the Outer Planes...."
Funny how I thought I was making up the whole "point of no return", only to find out that is exactly what canon calls it... Anyway, if this is the case, there is not
**currently a plot hole. The highlighted part above aligns with what I said: the issue is with the souls compatibility with the material plane. Hence why spells that have caveats on the soul's ability to return won't work (assuming everyone in the party has passed the point of no return). So Revivify is fine so long as the body remains in tact and the person died recently. You could also stretch things a bit and claim other raise dead spells might work as long as revivify WOULD also work in the same scenario.
However, non of this changes my Suggestion.
At issue is that most people will take the game at face value. Whether you know D&D lore or not, players will look for consistency, build expectations and discover discrepancies. Right now there is an apparent discrepancy creating the feeling of a plot hole around how simple tadpole removal should be. We have a scroll of true resurrection, a spooky creepy skeleton that can "cleave soul to flesh" when we die and we see tadpoles emerging left and right from all the true souls we are killing. The simple solution is apparent enough that people are posting about it in forums. (Per your first comment). So even if it is not a plot hole, I still feel it needs to be explained in game. My solution and wording may be overly simple, but at least I have Halsin's voice admit he doesn't know the specifics, only the important parts. Enough to convince you that head-smash is probably a bad idea and not one worth entertaining.
Anyway, I appreciate your feedback, especially the video link showing the tadpole emerge from a companion.
That brings me back to the point where the PCs simply should not die.
Let them stay incapacitated after the third failed save without the option to help them back up (too weak due to blood loss to get back into the fight or somesuch).
This sadly came to quite strong colision with curent rules ...
How would you explain difference between stabilized and unstabilized party member ... slightly and hardly incapacitated?

I actually really like the idea of just not dying (outside scripted sequences), but I wouldn't rely on it to solve the issue. My hope is that the game will have difficulty settings and some of those (Easy) will prevent companion death and others (hard) will allow it. Not sure what the default setting should be. But I feel the problem should be solved independently of death protections. It will be nice when they are in place, but the solution shouldn't rely on them.
As for rules... I think most users would chalk it up to convenience. Right now you don't have a choice but to revive them. If you are playing multiplayer., the game wont let you rest again (Partial or Long) until all Player Controlled Characters have been revived. And in single player, while you can rest when NPCs are dead, if someone dies and you decide you don't want to revive them (Wyl is useless... let him rot), you have to do it anyway. Why? Because you can't ask a dead party member to stay at camp and free up the slot for someone else. So things may change, but right now the game gives us no choice but to revive everyone. (Or reload and ensure they dont die). Why make people jump though hoops to do the inevitable?
Back to death protections, I think it would be good if Larian did something with Gale so that death isn't necessary for his story arc. Whether he has just died when you first meet him and you save him or he has a non-removable cursed item like shadowhearts weapon that causes him to turn to stone when reaching 1 HP. If they fixed that, then allowing no-death-for-companions would be a valid difficulty setting as well as avoiding perceived plot holes regarding the tadpole problem. In harder difficulties where companions die, you are still covered by the explanation I proposed placed in halsin's journal to prevent the feeling of plot holes.
I think that if the game is going to show us a scene of this happening, then the characters should talk about that scene. It's the obvious solution-not necessarily good, but obvious-and the fact no one voices it does make it harder for the players to tell if this is a mystery the game is presenting, or a mistake in writing that hasn't been accounted for.
Agreed that it would be nice if we learned about it and understood what people were thinking and not thinking through observed discussion. The catch is, not everyone will have much to say on it. If your party consisted of Shadowheart, Astarion and Wyl... not sure what those three would have to offer. You can also choose to play the game solo, which means you won't learn things that are only discovered through discussion with companions. Conversations would be great, but Larian still needs to explain things in a book as a fall back. And if budget and time are critical, I would start with the book and maybe add some conversation later, time permitting.
** A bit ironically... in this video, we see the parasite leave a companion's body within moments of death... that means technically we could cast revivify on the dead companion after the parasite left and they would be free to go... But obviously the game wont let us because... plot armor. So plot holes might be coming, but that content isn't officially in the game yet. I'm hoping they update the scene and remove the parasite stomp or go full decapitation after the initial stab, making revivify a non-option while explaining why the tadpole emerged...