Just been reading your excellent post linked above. As I understand it, I therefore could say "they are my bags" "de är mina väskor", if I'm emphasising 'my/mina', and referring to bags that have already been mentioned in a conversation (e.g. "Are they Bob's bags?" "No, they are my bags")
If, however I'm introducing my bags to the conversation for the first time, then the 'det är' construction at the start of the sentence is independent of the subject (the bags), and I should use "det är mina väskor".
This kinda makes sense to me now (I hope), but I don't think I'll ever get the hang of saying 'it is my bags' in the heat of the airport queue.
The conversation about those bags would still be Är det Bobs väskor? Nej det är mina väskor, unfortunately. Saying de there sounds unnatural. But if you skip väskor in the second sentence you get to say de: Är det Bobs väskor? Nej de är mina. Then you've changed it from a new topic to something known.
I know it's counter intuitive, I can't help thinking the English usage here is really weird.