Solid wood parquet on underfloor heating

"I'm just afraid that it will spread, for example, under the impact sound insulation."
Moisture can run into the joints, yes. But if that is not a permanent condition, it will dry out again without damage. I don't see any dramatic disadvantage regarding the floating floor.

"If one ever has to replace a plank because, for example, the top layer comes loose, etc., isn't that easier with glued flooring?"
"Replacing a plank" hardly ever happens, according to my experience. Technically, I think both are not easy, glued or floating... doesn't matter...

Personally, the surface treatment seems more important to me: oiled or lacquered?
Lacquered material looks cleaner in the first years than oiled (because almost no dirt sticks), but once the first scratches occur, it looks noticeably worse (because stains cannot be removed like with oiled material)... and scratches will happen eventually...

I imagine replacing glued-down planks to be significantly more difficult. After all, they are fully adhered if the installer has done their job properly. To get them out, you would probably have to damage the neighboring planks and replace them at the same time. Floating installation could (I’m not saying it’s recommended) involve cutting through the tongues of the click system on the affected plank (with a multitool, for example) and then gluing the new plank into the gap—well, then you do have to glue it after all. That should work reasonably well. With glued flooring, you also have the adhesive residues on the floor that you must remove completely in order to lay it flat again, which in my opinion is more difficult, but I am not a professional floor layer.

Floating, one could (I'm not saying it's recommended) cut through the feathers of the click system on the respective plank each time (with a multitool, for example) and then glue the new plank into the gap - well, then indeed - glue it in.

You do the same with glued parquet. Cut out and replace.

Both options are crap and are only done if the parquet is really ruined. Which with normal use might happen after 40 years (?).

Hi everyone, it will probably be installed floating. Due to the low wear layer of 2.5 mm and ultimately the strong persistence of my floor fitter, we're doing it floating now.

For what reasons? I don't understand. I just see the floor fitter saying "I don't feel like it." Or is the floor fitter's answer to the why the typical army answer: That's how it is!!!

Is floating installation disadvantageous if water gets on the parquet (we also have it in the dining area)? For example, a glass could tip over there. I'm just afraid that it will then spread, for example, under the impact sound insulation.

As with any floating floor (vinyl, design floor, laminate, etc.) it's harmless if you wipe up the liquid promptly and it's not a whole bucket (but even then it will dry again).

You do the same with glued parquet. Cut out and replace.

Sure, but wouldn’t you also say that the process is more elaborate with glued planks? After all, they stick firmly over the entire surface, meaning you either have to "chisel out" the entire plank bit by bit, or at least destroy an adjacent one to get a starting point from the long side under the plank, and then you have to remove glue residues from the floor afterward to get it as even as possible.
With floating floors, you simply cut the connection to the other planks and then just lift them out.

With "glued," you have to remove the wood and the glue and then glue in a new piece. Then the surface must be sanded and treated. If the floor is lacquered, you basically have no chance of repairing it satisfactorily. If it is oiled, it might work. With "floating," the inserted piece must be connected to the old floor at the edges (tongue and groove) ... which in most cases is probably not possible, - at least not by a "normal" craftsman.

No, the connection with tongue and groove no longer works then. The new plank must be glued, but if you work carefully when cutting out, you can keep the tongues of the adjacent planks intact and then cut off the ridge (if present) there with a chisel. Then you can also trim the lower edge of the groove on the new plank and thus only glue the planks together at the seam. This is also done during the initial installation, for example on edge pieces, when due to a frame or something similar you cannot insert from the back last but have to make the connection to the top row from the front last. Then you glue there as well even with a click system, but only along the touching edge. With a little practice, this is also doable for a not completely unskilled amateur.