"Sie hat ihren Sohn und ihren Mann allerdings seit über zehn Jahren nicht mehr gesehen."
As a native speaker I can say that order of words is awkward although not outright wrong. I would prefer to be taught elegant German and not just "not incorrect" German. Solution one: omit the "allerdings" - the sentence is long anyway. Solution two: Allerdings hat sie ihren Sohn und ihren Mann seit über ...
As a native speaker, I would use duo's example exactly like that within a longer anecdote about some woman. 'allerdings' in the middle of the sentence works fine.
'Morgen koche ich uns Spaghetti. Ich weiß allerdings noch nicht, was es übermorgen gibt.'
Yes and no. German has a very flexible sentence structure. Therefore the sentence is not wrong. If you place the "allerdings" in the middle of the sentence it usually would be placed right after the conjugated verb: Sie hat allerdings ihren Sohn und ihren Mann seit ... This is what you did in your your example: Ich weiß allerdings nocht nicht ...
I counter-checked your explanation in my head and it seems to be a rule indeed. But for this very example: replace allerdings with 'trotzdem schon' for (maybe a bad) example and I would put it in the same place in duo's example. I find it appropriate, but that might just be my sprachgefühl and in that case you might very well ignore my enquiry.
She has certainly not seen her son and her husband in over ten years. - Is that acceptable?
Why is "allerdings" used here (definition given is "absolutely"), and not "jedoch" ("however")?
"Seit" is used in German as "for", like "I've been living in Germany for two years" = "Ich lebe in Deutschland seit zwei Jahre".
Otherwise it means "since".