*"They had already ten children" is ungrammatical because an adverb, in general, cannot intervene between a verb and a direct object.
The sentence below, however, is OK:
"They have already had ten children"
That's because the first "have" is an auxiliary, not a lexical verb. An auxiliary does not take a direct object.
I think you may be confusing different uses of the same words.
As clever as I am, I couldn't repair my car.
This looks like an adverbial phrase meaning "Even though I am very clever"
He is as clever as I am
This looks like a complement.
The words are the same, but the intonation would likely be different, with different stress.
In this case, 'comprehend' means "to encompass," from Latin roots that mean "to hold completely."
Thus, the meaning of the phrase what such a purpose must comprehend in this convoluted, 71 word sentence (typical for Jane Austin and other writers of the time) is that he [Darcy] was grave and thoughtful at the inn only because of his decision to follow her [...
There is an article from Lund University in Sweden that explains:
The inverted order only occurs when the whole clause is affected by the initial negation/restriction. Thus, there are cases where what appears to be an initial negative element does not trigger inverted word order.
In no time at all, the two doctors had more patients than they ...