Does examiner know that a candidate had extra time?

yes agree with others, the examiner would not usually know but if they did know, they. couldn't allow that to have any bearing.

I examine speaking candidates in MFL (see many posts from me!) and sometimes a candidate qualifies for extra prep time. Obviously I know this and in some cases the speaking examiner marks the assessment; but there is no way I would take it into account in any way. For the most part, the assessments are sent away to be marked by a different examiner and obvs they would not know.

Equally (I've said this before but I think it bears repeating tho it is not on the same topic really) if a candidate sits foundation paper, that fact does not appear on their GCSE certificate. The grade is simply recorded as 4 or 5 or whatever, which might reassure people.

No disadvantages. Your DD doesn’t have to use the extra time but it’s there if she needs it. Invigilators obviously know, but not those who mark the number papers.

None at all - we have no idea unless they have a scribe or word processor.

No, there is no disadvantage.
The examiner will apply the mark scheme, as they would to any other script.
The examiner mostly should not be aware - I can think of no reason why the exam board would tell them as it would increase the possibility of bias. There are some accommodations where the examiner can't help but be aware (e.g. if they're typed because the child has been granted use of a keyboard). But again, there's an onus on the examiner to apply the mark scheme objectively, not make assumptions.
Extra time can be critical in ensuring a level playing field. If she has qualified, she needs it, and should use it.
Also, accommodations are not recorded in any way on the exam certificates - the certificate looks the same as any other.
About the only situation where I think it can have a drawback is if there are 2 long papers in the same day. In that situation, the total time including extra time could lead to fatigue on the second paper. there are rules whereby it can be carried to the next day but they have to be extreme to be used (from memory 6 hours total time, but it might have changed)