How to understand “as was inevitable” in this context

Question

Harry watched them go, feeling slightly uneasy. It just occurred to him that Mr and Mrs Weasley would want to know how Fred and George were financing their joke shop business when, as was inevitable, they finally found out about it. Giving the twins his Triwizard winnings had seemed a simple thing to do at the time, but what if it led to another family row and a Percy-like estrangement?

_{Harry Potter and the Order of the Pheonix}

I think as was inevitable can be taken as "as it was inevitable, meaning because it was inevitable. Is my understanding correct?

Answer 1

As you may know, you can move as was inevitable to the end without change of meaning:

... when they finally found out about it, as was inevitable

and it can be paraphrased "which was inevitable". The clause comments on the assertion in the other clause.

It is not uncommon.

Compare:

When the art thieves entered the museum via the storm sewer, as must have been the case since the cameras at all entrances show nothing, they found themselves in the basement in an area where controversial works no longer exhibited are kept.

Answer 2

It just occurred to him that Mr and Mrs Weasley would want to know how Fred and George were financing their joke shop business when, as was inevitable, they finally found out about it.

In fact the phrase is adverbial. It qualifies the verb 'found out'. See the following:

It just occurred to him that Mr and Mrs Weasley would want to know how Fred and George were financing their joke shop business when they inevitably found out about it.

This just avoids having two adverbs, 'finally' and 'inevitably' next to each other - although that would have been possible - and adds emphasis.