... (In the corner there is a comfortable couch.), I am really suprised to read more and more texts where the subject (here: there) is left out.

It can't be correct to describe "there" as the subject. I would say it's just part of a fixed phrase, "there is", but perhaps someone else can explain it better.

I wouldn't call "there" the subject of the sentence, and I find the suggested sentences to be more German-oriented (inverted word order) than the "standard" English S+V+* word order. In both cases, there is a prepositional phrase before what is in standard word order the "first part" of the sentence. (The two sentences in the OP aren't necessarily incorrect, although I wouldn't expect to find them standing by themselves without having a reason for inverting the word order, such as Nixda has given in #4 with the description of a room.)

For me, "couch" remains the subject of the sentence. In its simplest form, the sentence would read "a couch is in the corner."

I'm not the best at terms in grammar, but M-W has this to say about "there" as used in this way:

pronoun

• —used to introduce a sentence in which the subject comes after the verb There is a person outside.

It should also be

So maybe you can help me clarifying whether

#5 there

pronoun (M-W)

I've never seen "there" classified as a pronoun before, but I suppose this is reasonable if it is regarded as a placeholder for the noun that is the subject. (Of course M-W also has it as an adverb.)

But as you (Suney) have observed, and others have said, an expression of place like "In the corner" can sometimes be followed simply by is, stands, hangs, etc., without "there".

I teach my students a strict word order (subject - verb - object)

In the corner there is a comfortable couch.

Gerade das ist doch ein Beispiel für eine invertierte Satzstellung. Das Subjekt ist natürlich nicht "there", sondern "a comfortable couch", und das steht in diesem Fall ganz hinten.

I would tend to regard

(1) "In the corner there is a comfortable couch."

and

(2) "In the corner is a comfortable couch."

as two different grammatical constructions.

(1) is an example of an existential clause, where there is a "dummy pronoun".

(2) is an example of subject–complement inversion.

I am not a teacher, but I suspect it is a good idea to keep quiet initially about the second construction (subject–complement inversion) when teaching English to German native speakers – because the construction is very restricted in English and much more common in German.

However, I would say it is the standard construction, for instance, when describing pictures.

Rather than being simply a matter of word order, I would say it is a matter of "information packaging" or "thematic structure": the inversion is used (restrictedly) when the grammatical complement (consisting of a prepositional phrase) is the topic of the clause and the subject represents the information being imparted about that topic.

So, when describing a picture, you can answer the implicit questions "what is in the foreground?"etc.):

In the foreground is ...; in the background is ...).

The construction can also be used with verbs like stand or lie, which incidentally express orientation."

The construction is probably not appropriate if the verb contains significant novel information (in addition to existence and orientation); so (when describing a picture or scene) you can say

"At the desk sits a man."

"On the desk is a book. "

"Beside the desk stands a grandfather clock. "

"On the wall hangs a picture of a stag."

but you would not say

* "At the desk reads a man."

* "On the desk crawls an ant."

There are however examples of such inversion with verbs other than is, stand, etc. where it is difficult to specify what verbs are allowed:

"... two hours later came the news that they had been found alive."

This does not work if came is replaced by arrived.

I would say inversion is fairly formal and possibly used more widely in literary texts:

"At the entrance stood an empty cart."

"To the left lay darkness...

"At its bottom ran a hurrying stream "

RE #7: (Of course M-W also has it as an adverb.)

Yes, it's one of those "multipurpose words, but their definition and examples for "there" as a pronoun fit this situation most closely.

RE #9: Agree. In a sense, the inverted word order is a sort of "false friend" -- however, it's not incorrect per se in English, but we use it much less often than in German. I agree that German-speakers learning English should first master the standard S-V-(O) word order in English before doing a bunch with inverted word order.

It's for similar reasons that I believe that the inverted word is used far too often in translations done by NESs as well.

#10 ... but their definition and examples for "there" as a pronoun fit this situation most closely.

I know. I was trying to make it clear that I wasn't accusing M-W of describing "there" only or mainly as a pronoun.