My understanding is that once a lead unsubs, it is recorded to a Master Table behind the scenes. In theory, then if a lead matches that email again, it will not be sent. However, this was intended for deletion of records.
I know from dupe processing that if one lead is unsubbed, but another is not, the lead will still get sent an email under several circumstances. I believe in your case, you could definitely send them another email, especially since you have dupes in different LPs.
I would TEST this with your own records to see how it works.
As for the legal issues, that is for a lawyer. I suspect the company could be held liable as a whole, not just the BU. You could argue that a separate corporate division/wholly owned sub can have a separate relationship and since they subscribed to the others, this was ok. But I wouldn't stake my job on that w/o speaking with a lawyer.
As for ethics, I would say it is only unethical if your unsub page implied the lead could unsubscribe from ALL company emails, not just the one BU. So you should make that clear if you have separate unsub pages.
Also, this depends greatly on how you have structured your sub mgmt system and the ability for the Default to see all/none of the leads.