My initial thoughts on the abductor(s?) being in the house for 40ish minutes would be that they were trying to get NG to do something that she was not cooperating with, whether that was give them money/ bitcoin, call SG, call other kids to extort funds, etc. Then they got frustrated and switched to Plan B, which was to abduct her and send the ransom note. I don't feel as that was their initial intent. I also believe this would explain how she was injured and dripping blood on the doorstep, they injured her in their frustration.
My 2 cents: this wasn't completely random but it may be a stranger to the family. They knew who the target was related to and/ or that she had means herself. I think they were watching her for a long while, possibly in the house before she returned from the family dinner. Definitely amateur with the lack of planning re: doorbell cam. I also think it was two people, one being the driver/ lookout. Unfortunately I don't think NG was alive long after leaving the house and I do believe she was dumped somewhere fairly early on.