A rare structure need to be explained in terms of grammar: Have+ noun+ been [closed]

Dear Enlish Language & Usage Exchange members:

I was reading a text and came across a remarkable structure for me. Here is the text:

It would, I suppose, be universally agreed that seldom, if ever, have the powers of critical philosophy and of historical scholarship been united in one person to the same extent as they have been in Lovejoy.

So members, as is seen, in the text after the word of "have" there is a bunch of nouns and then "been". To focus on a problematic part by shortening the text, I would like to say what I mean:

"It would be universally agreed that the powers of critical philosophy and of historical scholarship have been united in one person to the same extent as they have been in Lovejoy." (I remove the "I suppose" and "seldom, if ever" because I am under the impression that they are not really important).

So does what I understand is true? I mean I think this structure is present perfect tense and the author wanted to say "have been bla bla" but instead he said "have bla bla been" making the structure changed. Is that right? If it is, could you explain to me for what this structure is used? And what this structure is called in terms of grammar? For example is it inversion or somewhat different? If I understand wrongly could you explain to me the true meaning?

By the way friends, I am not a native English speaker. Therefore I understand the English text according to grammar. If you are a native speaker, ıt might be easy to understand. But for me it is not. I am trying to get used to these usages.

Thanks.

(I remove the "I suppose" and "seldom, if ever" because I am under the impression that they are not really important).

But they are really important.

It's a common sort of inversion.

When do we use inversion? Of course, we use inversion in questions. You can read more about this here. But we also sometimes use inversion in other cases, when we are not making a question.

1: When we use a negative adverb or adverb phrase at the beginning of the sentence.

Usually, we put the expression at the beginning of the sentence to emphasise what we're saying. It makes our sentence sound surprising or striking or unusual. It also sounds quite formal. If you don't want to give this impression, you can put the negative expression later in the sentence in the normal way:

Seldom have I seen such beautiful work. ('Seldom' is at the beginning, so we use inversion. This sentence emphasizes what beautiful work it is.)

I have seldom seen such beautiful work. ('Seldom' is in the normal place, so we don't use inversion. This is a normal sentence with no special emphasis.)

Perfect English Grammar