comma after introductory clause [when, though, conjunction]

[Julianus]

Hello.

1a. When they thought they were alone (though they were being taped by a secret camera) their faces twisted into vivid mixes of uncomfortable feelings. (Korea university entrance exam)
1b. When they thought they were alone (though they were being taped by a secret camera), their faces twisted into vivid mixes of uncomfortable feelings.

When 'when-clause' is used before the main sentence, shouldn't 'the comma' is used between 'adverbial clause' and 'the main sentence'? Therefore, shouldn't the comma be used between 'camera' and 'their faces'? If not, when 'when-clause' is used before the main sentence, is the comma just optional? In other words, are both 1a and 1b all correct?

Thank you always~.

 

[Cagey]

Hello Jullianus.

I think only 1b is correct. If we removed the parenthetical clause from 1a, we would have:
When they thought they were alone their faces twisted into vivid mixes of uncomfortable feelings.
As you say, the adverbial clause should be set off by a comma.

We discussed a similar problem here: Comma after parentheses.